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.

1 files changed, 8 insertions, 0 deletions

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index ef83c22155..5704547689 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -265,6 +265,12 @@ Many `:help` docs are autogenerated from (C or Lua) docstrings. To generate the
 make doc
 ```
 
+To validate the documentation files, run:
+
+```bash
+make lintdoc
+```
+
 If you need to modify or debug the documentation flow, these are the main files:
 - `./scripts/gen_vimdoc.py`:
   Main doc generator. Drives doxygen to generate xml files, and scrapes those
@@ -282,6 +288,8 @@ If you need to modify or debug the documentation flow, these are the main files:
   src/nvim/options.lua  =>  runtime/doc/options.txt
   ```
 
+- `./scripts/lintdoc.lua`: Validation and linting of documentation files.
+
 ### Lua docstrings
 
 Use [LuaLS] annotations in Lua docstrings to annotate parameter types, return