Categorize xplr messages in the docs

[sayanarijit]

Xplr works by using a simple message passing system.
Messages are sent to a running xplr session by binding them to some key, using --on-load, or by calling lua functions or bash scripts (using messages of course).

So, there's a long list of messages documented in https://xplr.dev/en/message, which might be overwhelming.

One way we can tackle this is by putting them into separate categories, e.g. navigation, debugging, sorting, filtering, etc.

[sayanarijit]

Done in #458