Update XHR event pages

[Elchi3]

This PR updates the XHR API for new-style event pages. (see the template)

Here, events aren't installed under XMLHttpRequestEventTarget anymore. (see the discussion in the BCD issue). Instead, XMLHttpRequestEventTarget is made a page that explains how it basically a mixin for XMLHttpRequest and XMLHttpRequestUpload. It is technically not a mixin, that's why the page is kept (and it is exposed to the web). The inheritance of it is the important information.

Commands used to move pages around:

yarn content move Web/API/XMLHttpRequest/onreadystatechange Web/API/XMLHttpRequest/readystatechange_event  
yarn content delete Web/API/XMLHttpRequestEventTarget/onabort --redirect=/en-US/docs/Web/API/XMLHttpRequest/abort_event   
yarn content delete Web/API/XMLHttpRequestEventTarget/onerror --redirect=/en-US/docs/Web/API/XMLHttpRequest/error_event  
yarn content delete Web/API/XMLHttpRequestEventTarget/onload --redirect=/en-US/docs/Web/API/XMLHttpRequest/load_event 
yarn content delete Web/API/XMLHttpRequestEventTarget/onloadstart --redirect=/en-US/docs/Web/API/XMLHttpRequest/loadstart_event
yarn content delete Web/API/XMLHttpRequestEventTarget/onprogress --redirect=/en-US/docs/Web/API/XMLHttpRequest/progress_event

BCD: mdn/browser-compat-data#13799

[github-actions]

Preview URLs

• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/abort_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/error_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/getAllResponseHeaders
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/load_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/loadend_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/loadstart_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/progress_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/readystatechange_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/timeout_event
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest/upload
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequest
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/XMLHttpRequestEventTarget
• https://pr12896.content.dev.mdn.mozit.cloud/en-US/docs/WebAssembly/Loading_and_running

Flaws

Note! 9 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/API/XMLHttpRequestEventTarget
Title: XMLHttpRequestEventTarget
on GitHub
Flaw count: 17

• macros:
  • /en-US/docs/Web/API/XMLHttpRequestUpload does not exist
  • /en-US/docs/Web/API/XMLHttpRequestUpload does not exist
  • /en-US/docs/Web/API/XMLHttpRequestUpload/abort_event does not exist
  • /en-US/docs/Web/API/XMLHttpRequestUpload/error_event does not exist
  • /en-US/docs/Web/API/XMLHttpRequestUpload/load_event does not exist
  • and 5 more flaws omitted
• bad_bcd_links:
  • no explanation!
  • no explanation!
  • no explanation!
  • no explanation!
  • no explanation!
  • and 2 more flaws omitted

---

URL: /en-US/docs/Web/API/XMLHttpRequest
Title: XMLHttpRequest
on GitHub
Flaw count: 2

• macros:
  • /en-US/docs/Web/API/XMLHttpRequestUpload does not exist
• bad_bcd_links:
  • no explanation!

---

URL: /en-US/docs/Web/API/XMLHttpRequest/readystatechange_event
Title: XMLHttpRequest: readystatechange event
on GitHub
Flaw count: 1

• bad_bcd_queries:
  • No BCD data for query: api.XMLHttpRequest.readystatechange_event

---

URL: /en-US/docs/Web/API/XMLHttpRequest/upload
Title: XMLHttpRequest.upload
on GitHub
Flaw count: 1

• macros:
  • /en-US/docs/Web/API/XMLHttpRequestUpload does not exist

---

URL: /en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data
Title: Fetching data from the server
on GitHub
Flaw count: 1

• broken_links:
  • Link points to the page it's already on

External URLs

URL: /en-US/docs/WebAssembly/Loading_and_running
Title: Loading and running WebAssembly code
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequestEventTarget
Title: XMLHttpRequestEventTarget
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest
Title: XMLHttpRequest
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/readystatechange_event
Title: XMLHttpRequest: readystatechange event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/loadend_event
Title: XMLHttpRequest: loadend event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/load_event
Title: XMLHttpRequest: load event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/upload
Title: XMLHttpRequest.upload
on GitHub

No external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/loadstart_event
Title: XMLHttpRequest: loadstart event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/progress_event
Title: XMLHttpRequest: progress event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/abort_event
Title: XMLHttpRequest: abort event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/timeout_event
Title: XMLHttpRequest: timeout event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/error_event
Title: XMLHttpRequest: error event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/XMLHttpRequest/getAllResponseHeaders
Title: XMLHttpRequest.getAllResponseHeaders()
on GitHub

No new external URLs

---

URL: /en-US/docs/Learn/JavaScript/Client-side_web_APIs/Fetching_data
Title: Fetching data from the server
on GitHub

No new external URLs