fix(blog): apply trailing slash to blog feed

[OzakIOne]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

tools like Algolia that use the feed files may behave unusual if trailing slash is enabled and not applied to links

Test Plan

feed.test.ts => has feed item for each post trailing slash

Deploy preview ??

Test links

rss.xml

Related issues/PRs

Fix #9829

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 66	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 64	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 68	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 66	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 69	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 73	🟢 100	🟢 100	🟢 90	🟠 88	Report

[github-actions]

Size Change: 0 B

Total Size: 992 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	75.4 kB
website/build/assets/css/styles.********.css	114 kB
website/build/assets/js/main.********.js	765 kB
website/build/index.html	37.9 kB

_{compressed-size-action}

[OzakIOne]

Found a todo

docusaurus/packages/docusaurus-utils-common/src/applyTrailingSlash.ts

Lines 27 to 30 in d7a7cae

	// TODO deduplicate: also present in @docusaurus/utils
	function addTrailingSlash(str: string): string {
	return str.endsWith('/') ? str : `${str}/`;
	}

I think we can do it in this PR however should we keep the one in utils-common or utils ?

[slorber]

• utils is Node.js only utils
• utils-common is Browser + Node.js

This code is env agnostic so it should rather be in utils-common, but is not because this shared package was introduced later. I'd prefer if you did this refactor in another PR because this will impact various files unrelated to this feature.

[ilg-ul]

Thank you!