Contributes to
https://gitlab.com/gitlab-org/gitlab/-/issues/345474#note_1447561145

* Disallow `.md` extension usage
* Add todo list to fix offences

Auto-corrected via:
  rubocop -a --only Style/RedundantFreeze ee haml_lint

Match the following use cases too:

= Rails.application.routes.url_helpers.help_page_path("invalid/path")

This commit also improves the speed of this lint rule by caching the
node pattern.

This simplifies the setup:
* Removes custom `require` in rake task
* Removes `require`s and `production?` guard in linter
* Adds `require` to .haml-lint.yml as suggested in HAML linter docs

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/358703

**Problem**

Code that ends with a `do` block definition is ignored by parser. The
linter can skip some of broken links because of that.

**Solution**

Add a workaround for this case.

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28570, we
moved the `haml_lint` directory out of an auto-load path to avoid a
startup failure in production
(https://gitlab.com/gitlab-org/gitlab-foss/-/issues/62125). However,
`inline_javascript.rb` used `require_dependency`, which is meant for
auto-loading and should not be mixed with `require`. As a result, the
linter registry was being reset twice: once for the `require_dependency`
call and once for the `require` call. This effectively dropped the
inline JavaScript linter.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/342479

Changelog: fixed

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/328203

**Problem**

NodePattern#match returns the first matching subnode and ignores other
matches

**Solution**

Apply NodePattern#match for all node descendants

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/208354
Previou implementation MR:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35351

**Problem**

The documentation link linter checks links defined by `help_page_path`
helper, however it ignores links from `help_page_url`.

**Solution**

Extend the pattern to match both `help_page_path` and `help_page_url`
helper methods

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/218764
Follow-up for:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39717
After merging:
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39715

* Revert an approximate match in the documentation linter
* Revert an allow_failure: true behavior
* Update a regular expression to remove false-positives in the linter

* Remove starter-only
* Remove core-only
* Remove premium-only
* Remove ultimate-only
* Remove ultimate
* Remove premium
* Remove starter
* Remove core

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/208354

We want to validate if there are any broken links and dead anchors to
documentation pages.

Extra:
* Extract anchor generation logic into Gitlab::Utils::Markdown module

Files in lib will be eager loaded and hence will require haml_lint to be
loaded. Since this is only a development dependency, we can't assume
this gem will be available in production, so it should never be loaded
in production.