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.

scripts/lintdoc.lua

@@ -0,0 +1,20 @@
+#!/usr/bin/env -S nvim -l
+
+-- Validate vimdoc files on $VIMRUNTIME/doc, and test generating HTML docs.
+-- Checks for duplicate/missing tags, parse errors, and invalid links/urls/spellings.
+-- See also `make lintdoc`.
+--
+-- Usage:
+--   $ nvim -l scripts/lintdoc.lua
+--   $ make lintdoc
+
+print('Running lintdoc ...')
+
+-- gen_help_html requires :helptags to be generated on $VIMRUNTIME/doc
+-- :helptags checks for duplicate tags.
+vim.cmd [[ helptags ALL ]]
+
+require('scripts.gen_help_html').run_validate()
+require('scripts.gen_help_html').test_gen()
+
+print('lintdoc PASSED.')