Fix nested redirects

[dgarcia360]

Motivation

There was an issue with nested redirections after the migration.

Here's an illustrative example extracted from the redirects file:

Before:

https://docs.scylladb.com/getting-started/types/index.html -> https://docs.scylladb.com/stable/getting-started/types https://docs.scylladb.com/ /stable/getting-started/types.html -> https://docs.scylladb.com/stable/cql/types.html

Now:

https://docs.scylladb.com/getting-started/types/index.html: https://opensource.docs.scylladb.com/stable/getting-started/types https://docs.scylladb.com/ /stable/getting-started/types.html: https://opensource.docs.scylladb.com/stable/cql/types.html

Note that what was previously a nested redirect, now it breaks after incorporating the new domain name opensource.docs.scylladb.com.

This pull request fixes this issue. It also fixes some redirects related to scylla-monitng which where returning 404 errors. In those cases, I opted to define a more generic redirect to https://monitoring.docs.scylladb.com/

[annastuchlik]

Thanks, looks good. Let's merge it and see if it solves all the problems with redirections.