build(docs): separate lint job to validate vimdoc #27227

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.

.github/workflows/docs.yml

@@ -7,6 +7,8 @@ on:
       - 'src/nvim/eval.lua'
       - 'runtime/lua/**.lua'
       - 'runtime/doc/**'
+      - 'scripts/gen_vimdoc.py'
+      - 'scripts/gen_help_html.lua'
 jobs:
   docs:
     runs-on: ubuntu-latest
@@ -30,3 +32,6 @@ jobs:
             echo "::error::The doc generation produces the following changes:"
             git diff --color --exit-code
           fi
+
+      - name: Validate docs
+        run: make lintdoc