Handling for additional metadata

What does this MR do and why?

Conditional rendering for parts of the page, based on front matter attributes: noindex, toc, and feedback.

This copies over this existing functionality and should behave the same way: https://docs.gitlab.com/ee/development/documentation/metadata.html#additional-metadata,

These front matter options are not included here:

• searchbar: we won't need this on the new site; it is currently unused in regular content.
• redirect_to: redirects will be handled in #29 and #30

How to set up and validate locally

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

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Run the dev server: make view
3. Add front matter like this to a page:

toc: false # hide the toc
noindex: true # include a noindex metatag
feedback: false # hide the help & feedback panel

These should behave as described here: https://docs.gitlab.com/ee/development/documentation/metadata.html#additional-metadata

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.