Motivation

Adds an option to override the multiversion redirect for the main domain.

It is particularly useful for redirecting docs.scylladb.com/manual to docs.scylladb.com/manual/stable instead of docs.scylladb.com/stable.

How to test

1. Open docs/conf.py.

2. Define a custom redirect in html_theme_options as follows:

   html_theme_options = {
       "redirect": "/manual/stable",
   }

3. Build the documentation using make multiversionpreview.

4. Verify that http://localhost:5500/ redirects to /manual/stable instead of /stable.