Update Endpoints documentation with helpful details. (#4033)

* Update Endpoints documentation:

- Describe how simple endpoints can be access directly via `/__data.json`.
- Describe how standalone endpoints can be given file extensions (`.json`).

* use more realistic endpoint, add index files, add src/routes prefix

* support tables

* combine __data.json docs with accept header

Co-authored-by: Rich Harris <hello@rich-harris.dev>

documentation/docs/01-routing.md

@@ -191,7 +191,7 @@ export async function post({ request }) {
 </form>
 ```
 
-If you request the route with an `accept: application/json` header, SvelteKit will render the endpoint data as JSON, rather than the page as HTML.
+If you request the route with an `accept: application/json` header, SvelteKit will render the endpoint data as JSON, rather than the page as HTML. You can also get the raw data by appending `/__data.json` to the URL, e.g. `/items/__data.json`.
 
 #### Body parsing
 
@@ -269,6 +269,15 @@ Most commonly, endpoints exist to provide data to the page with which they're pa
 
 > Support for streaming request and response bodies is [coming soon](https://github.com/sveltejs/kit/issues/3419).
 
+Standalone endpoints can be given a file extension if desired, or accessed directly if not:
+
+| filename                      | endpoint   |
+| ----------------------------- | ---------- |
+| src/routes/data/index.json.js | /data.json |
+| src/routes/data.json.js       | /data.json |
+| src/routes/data/index.js      | /data      |
+| src/routes/data.js            | /data      |
+
 ### Private modules
 
 Files and directories with a leading `_` or `.` (other than [`.well-known`](https://en.wikipedia.org/wiki/Well-known_URI)) are private by default, meaning that they do not create routes (but can be imported by files that do). You can configure which modules are considered public or private with the [`routes`](/docs/configuration#routes) configuration.

sites/kit.svelte.dev/src/lib/docs/client/docs.css

@@ -160,6 +160,7 @@
 .content table {
 	margin: 0;
 	margin-bottom: 2em;
+	max-width: var(--linemax);
 }
 
 .content section {

sites/kit.svelte.dev/src/routes/content.json.js

@@ -103,14 +103,10 @@ function plaintext(markdown) {
 		listitem: block,
 		checkbox: block,
 		paragraph: (text) => `${text}\n\n`,
-		table: () => {
-			throw new Error('TODO implement tables');
-		},
-		tablerow: () => {
-			throw new Error('TODO implement tables');
-		},
-		tablecell: () => {
-			throw new Error('TODO implement tables');
+		table: block,
+		tablerow: block,
+		tablecell: (text, opts) => {
+			return text + ' ';
 		},
 		strong: inline,
 		em: inline,