Split clients README file

[liorsve]

This PR -

• Splits the clients README file to two separate files, one listing only the advanced client features. This is to later allow for importing this separate features file directly to the website clients page, and avoiding the current multiplication of text both here and in the website repo's _index.md file in the clients folder (follow up for Adds a clients overview page valkey-io.github.io#166 (comment)).
• Replaces the link to the pull request in the CLIENT CAPA redirect part with a link to the docs (follow up for Adds a clients overview page valkey-io.github.io#166 (comment))

[liorsve]

@madolson @zuiderkwast

[zuiderkwast]

Sure, I'm fine with this. Sorry for the delay.

I don't particularly like the link text "this link" though, but that's not new as this PR only moves the content to a new place.