Split make compile to backend and frontend make targets

What does this MR do and why?

Split make compile to backend and frontend make targets.

This is useful when running the internal links checks locally, as we only need the backend to compile. Not compiling the JavaScript shaves off several seconds.

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/gitlab-docs/-/blob/main/doc/setup.md.

2. Run the following:

   make compile           # should be the same as before
   make compile-backend   # should only compile the HTML
   make compile-frontend  # should only compile the css and js

3. Running the following should not compile CSS and JS, thus making the check a bit faster:

   make internal-anchors-check
   make internal-links-check
   make internal-links-and-anchors-check

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.