Problem:
scripts/check_urls.vim manually matches urls in the help pages and then
synchronously checks them via curl/wget/powershell. This is extremely
slow (~5 minutes for Nvims runtime on my machine) and prone to errors in
how the urls are matched.

Solution:
- Use Tree-sitter to find the urls in the help pages and `vim.net.request` to
  check the responses.
- Add a `lintdocurls` build task and check it in CI (every Friday).
  - Reopens a dedicated issue if it finds unreachable URLs.
- Drop the old check_urls.vim script.

Problem:
"make lintdoc" is not validating vimdoc (:help) tags.

Solution:
- Call `lang_tree:parse()` to init the parser.
- Load netrw 🤢 explicitly, since it was moved to `pack/dist/opt/`.
- Fix invalid help tags.

- Move all generator Lua scripts to the `src/gen/`
- Add a `.luarc.json` to `src/gen/`
- Add a `preload.lua` to `src/gen/`
  - Add `src` to `package.path` so it aligns with `.luarc.json'
- Fix all `require` statements in `src/gen/` so they are consistent:
    - `require('scripts.foo')` -> `require('gen.foo')`
    - `require('src.nvim.options')` -> `require('nvim.options')`
    - `require('api.dispatch_deprecated')` -> `require('nvim.api.dispatch_deprecated')`

Summary: Separate the lint job (`make lintdoc`) to validate runtime/doc,
it is no longer as a part of functionaltest (help_spec).

Build (cmake) and CI:

- `make lintdoc`: validate vimdoc files and test-generate HTML docs.
  CI will run this as a part of the "docs" workflow.

- `scripts/lintdoc.lua` is added as an entry point (executable script)
  for validating vimdoc files.

scripts/gen_help_html.lua:

- Move the tests for validating docs and generating HTMLs from
  `help_spec.lua` to `gen_help_html`. Added:
  - `gen_help_html.run_validate()`.
  - `gen_help_html.test_gen()`.

- Do not hard-code `help_dir` to `build/runtime/doc`, but resolve from
  `$VIMRUNTIME`. Therefore, the `make lintdoc` job will check doc files
  on `./runtime/doc`, not on `./build/runtime/doc`.

- Add type annotations for gen_help_html.