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

There are a number of build tooling and minor problems with the website.
(Listed below)

**How did you fix it?**

Maintainers: I have marked this as draft for now due to the
Questions/TODOs/TBDs. Most of these I just implemented the fix that I
think is the most straightforward/reasonable. Feel free to
edit/merge/cherry-pick anything here as you see fit.

- [x] ~~The bundled TS type check is bugged (e.g.
[xterm-headless](https://yarnpkg.com/package?q=xterm-head&name=xterm-headless)
is reported as "no types")~~
- [x] ~~Fix: prioritize type entrypoints (`types` and `typings` fields
in `pacakge.json`, `types` condition) instead of normal (JS) entrypoints
when resolving the package.~~
- On second thought I think it is better to split this to another PR
(#5867)
- [x] Building the website fails on Windows due to [quirks of
child_process](https://nodejs.org/api/child_process.html#child_process_spawning_bat_and_cmd_files_on_windows)
on Windows
- [x] Fix: use `cross-spawn`, which `@yarnpkg/core` already depends on
- [x] There are a number of unused files/assets/patches/dependencyMetas
    - [x] Fix: delete them
- [x] The README is the default one generated by Docusuarus
    - [x] Fix: Update it to have slightly more useful info
- [x] There are a number of broken links, as reported by docusaurus
during `build`
- In particular, the [Release
Workflow](https://v3.yarnpkg.com/features/release-workflow) was not
ported to the new website.
    - [x] Fix: port the Release Workflow page from the old website
    - [x] Fix: Delete the extraneous "Publishing" page, for now
    - [x] Fix: change broken links to new URLs
- [x] ~~**Question**: What is the "Publishing" page? Is it supposed to
be a re-written Release Workflow page?~~
- [ ] There are also a number of broken links from other places in this
repo
- There are a total of 12 broken links (404 or not the intended page)
- 1 of them is to https://yarnpkg.com/advanced/sherlock, where there is
no short-term resolution (see #5859)
        - 2 of them are to https://yarnpkg.com/api, see below
        - 4 of them are in old blog posts. Let's leave them be
        - [x] Fix: Fix the other 5
- There are a few links to https://yarnpkg.com/features/zero-installs,
which 301-redirects to
https://yarnpkg.com/features/caching#zero-installs
- 2 of them are in old blog posts / CHANGELOG. Let's leave them be
        - [x] Fix: Change the others to the canonical URL
- There are a few links to package pages like
https://yarnpkg.com/package/eslint, which 301-redirects to
https://yarnpkg.com/package?name=eslint
        - [ ] **Question**: Should we change those to the canonical URLs
- [x] There is an extraneous TOC block in the
[Rulebook](https://yarnpkg.com/advanced/rulebook), a remnant of the old
Gatsby-based website
- [x] Fix: Remove it. Docusaurus adds a right sidebar/"On this page"
block automatically
- [x] The output during build is quite excessive. In particular, our own
commandLineHighlight plugin is spewing multi-line "fail to parse" errors
onto the console, which interferes with docusaurus' own progress bar
- [x] Fix: reduce the commandLineHighlight plugin output to only display
the command line that failed to parse
- ~~**Question**: This a not a very elegant solution. Are there better
alternatives?~~
    - I have an idea for a better solution. Will be a separate PR
- [x] There are two warnings during build:
    - `webpack` warns `js-yaml` on using `buffer`
    - [x] Fix: Explicitly disable the `buffer` polyfill as suggested
- `autoprefixer` warns `src/css/custom.css` on using
`text-decoration-skip` CSS property
    - [x] Fix: Use `text-decoration-skip-ink` instead as suggested
- [x] Add a dropdown to the navbar that links to the old websites
- [x] ~~**Question**: Should we use [versioned
docs](https://docusaurus.io/docs/versioning) for v4+?~~
- ~~**TBD**: While generating the CLI docs, run the CLI in JS instead of
via the OS shell to avoid spawning a terminal window~~
- ~~**TBD**: The API reference is completely removed. What should we do
about it?~~
- ~~**TBD**: Should we update `caniuse-lite` as suggested? Should we do
it regularly?~~

Closes #5667 (supersedes)
Closes #5860 (supersedes)
~~Closes #5894 (supersedes)~~
Fixes #5707
Fixes #5805
Fixes #5832
~~Fixes #5861~~
~~Fixes #5869~~
Fixes #5926

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