Allow /help to accept either page title in front matter or in Markdown

What does this MR do?

To help answer one of the questions at: gitlab-org/technical-writing-group/gitlab-docs-hugo#56, allow /help to properly render Markdown pages that have the title in front matter instead of in main Markdown content.

This allows /help to render Markdown files destined for a Hugo site, per: https://gohugo.io/content-management/front-matter/.

Validation instructions

1. Set up GDK on master branch of GitLab.
2. Check out this branch as your GitLab branch.
3. Open GitLab in GDK and turn off /help redirection: https://docs.gitlab.com/ee/administration/settings/help_page.html#redirect-help-pages.
4. Navigate to https://<gdk_url>/help/install/install_methods.html. The page will appear as you would expect.
5. In the gitlab codebase, modify doc/install/install_methods.md and remove the heading # Installation methods and add title: Installation methods to the front matter.
6. Reload https://<gdk_url>/help/install/install_methods.html and the page should look exactly the same.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product tier badge under the H1 topic title.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.