Re-enable DocumentationLinks in haml-lint

What does this MR do and why?

As a final step to complete #419673 (closed), this re-enables DocumentationLinks in haml-lint, and fixes the remaining issues that were either not caught in the earlier cleanups, or got introduced since then.

• Resolves Add '.md' extension to docs link helpers and up... (#419673 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before:

❯ bundle exec haml-lint -i DocumentationLinks
app/views/admin/application_settings/_organizations_api_limits.html.haml:7 [W] DocumentationLinks: add .md extension to the link: administration/settings/rate_limit_on_organizations_api

[..snip..]

ee/app/views/notify/user_associated_with_enterprise_group_email.html.haml:4 [W] DocumentationLinks: add .md extension to the link: user/enterprise_user/index

2194 files inspected, 45 lints detected

After:

❯ bundle exec haml-lint -i DocumentationLinks

2194 files inspected, 0 lints detected

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.