Co-Authored-By: Tim Otten <totten@civicrm.org>

Add in documentation on setting and retrieving contact settings using the Civi:: facade and also add in note about the deprecation of the ::setItem function

__Before__: The documentation explains ~5 common labels.  Gitlab actually has ~50
labels.

__After__: The documentation explains ~3 important labels and ~4 *prefixes*.
Each prefix is given with a definition and example, and there are links
to the full lists.

__Comment__: The link goes to https://lab.civicrm.org/dev/core/-/labels
though arguably https://lab.civicrm.org/groups/dev/-/labels might be better.
However, there a dozen or so labels which (rightly or wrongly) only show up
in `dev/core`, so `dev/core` gives the more comprehensive listing.

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

This table is not relevant for several install methods (Vagrant/Docker/Generic).

For Ubuntu/Debian, I think it's useful information, but it's a bit
overwhelming, and what we're basically doing is elaborating on a couple
words (*it needs to be a __recent__, supported version of Ubuntu/Debian -
which is specifically yadda yadda...*).

If one is oblivious to this content and runs the install script on an
unsupported version of Ubuntu/Debian, that should be safe -- since the first
thing it does is to verify compatibility.

Note: not trying to bury - the next commit will call this out a bit more.

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>