Generate CLI docs as a Docusaurus plugin (#6218)

**What's the problem this PR addresses?**

The current method of CLI docs generation is very inefficient because it
is synchronously blocking and memory-intensive. It can easily take up
40% of the time needed to start the dev server.

**How did you fix it?**

Created a Docusaurus plugin that generates the CLI docs. This has a
number of advantages over the existing method:

- It is async and happens in parallel to other Docusaurus plugins, thus
much faster in terms of total build time.
- It is written in TypeScript, so type-checked and has better DX
- It obtains the command definitions by running the TypeScript source
directly instead of going through a shell, saving overhead
- It writes the result to disk via Docusaurus APIs, saving memory

### Performance

```
# Before
Warm dev startup
  Time (mean ± σ):     68.887 s ±  1.508 s    [User: 114.795 s, System: 18.496 s]
  Range (min … max):   67.447 s … 71.328 s    5 runs

Cold dev startup
  Time (mean ± σ):     68.346 s ±  0.204 s    [User: 104.264 s, System: 15.938 s]
  Range (min … max):   68.195 s … 68.675 s    5 runs

Warm Build
  Time (mean ± σ):     110.748 s ±  0.781 s    [User: 139.832 s, System: 15.310 s]
  Range (min … max):   109.902 s … 111.528 s    5 runs

Cold Build
  Time (mean ± σ):     306.749 s ± 10.032 s    [User: 1588.512 s, System: 300.103 s]
  Range (min … max):   298.550 s … 322.576 s    5 runs
  
# After
Warm dev startup
  Time (mean ± σ):     42.410 s ±  0.546 s    [User: 60.527 s, System: 6.049 s]
  Range (min … max):   41.807 s … 43.265 s    5 runs

Cold dev startup
  Time (mean ± σ):     42.275 s ±  0.326 s    [User: 59.571 s, System: 5.824 s]
  Range (min … max):   41.760 s … 42.657 s    5 runs

Warm Build
  Time (mean ± σ):     85.861 s ±  2.109 s    [User: 95.237 s, System: 5.144 s]
  Range (min … max):   83.617 s … 88.499 s    5 runs

Cold Build
  Time (mean ± σ):     149.498 s ±  7.973 s    [User: 440.898 s, System: 15.700 s]
  Range (min … max):   140.074 s … 160.208 s    5 runs
```

### Index pages

I have also taken the opportunity to take the `@yarnpkg/cli` index page
and adapted it to the other binaries

### ⚠️ URL changes

Unfortunately, generating the CLI docs as a separate plugin makes the
original URL scheme conflict with the main docs plugin, so we either
have to

- move all generated CLI docs under a single path prefix (e.g. `/cli`)
- lose state (in particular, sidebar scroll state) when navigating
between pages for different binaries

I have opted to move everything under `/cli`

### Other Changes

The change causes a few visual changes to existing stuff:

- The previous page and next page links have been removed. Should be
easy to recreate but I don't feel like they have much value
- The sidebar is now sorted in lexicographical order
- The additional binaries' examples now include the `yarn` invocation,
and thus is properly styled
- Very minor, but the column widths in the options table have shifted
very slightly

### Future work

This PR is ready, but I am still experimenting with a few things that
may or may not make it into the PR.

- <s>I'm trying whether it is possible to use Docusaurus's watch
mechanism to hot rebuild the pages.</s>
- <s>Also, as you can see, there are practically no difference in warm
and cold startup time. Maybe checking mtimes can avoid some work?</s>


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Author: clemyan

.pnp.cjs

@@ -0,0 +1,14 @@
+releases:
+  "@yarnpkg/builder": patch
+  "@yarnpkg/pnpify": patch
+  "@yarnpkg/sdks": patch
+
+declined:
+  - "@yarnpkg/plugin-constraints"
+  - "@yarnpkg/plugin-exec"
+  - "@yarnpkg/plugin-interactive-tools"
+  - "@yarnpkg/plugin-stage"
+  - "@yarnpkg/plugin-typescript"
+  - "@yarnpkg/plugin-version"
+  - "@yarnpkg/plugin-workspace-tools"
+  - "@yarnpkg/cli"