Turn the index page of the documentation into a redirect to the user guide.

[JustusAdam]

What Changed?

Flips the index page of your documentation to a HTML that redirects to the user guide notion page.

Why Does It Need To?

This way we have a) a nicer landing page and also an easy-ish to remember link we can give people to the guide.

Checklist

• Above description has been filled out so that upon quash merge we have a
  good record of what changed.
• New functions, methods, types are documented. Old documentation is updated
  if necessary
• Documentation in Notion has been updated
• Tests for new behaviors are provided
  • New test suites (if any) ave been added to the CI tests (in
    .github/workflows/rust.yml) either as compiler test or integration test.
    Or justification for their omission from CI has been provided in this PR
    description.
• Remove the "push" trigger for the workflow after ensuring it works.