Document how to enable memcached

Hi,

I do not understand how to activate the caching feature. I checked the checkbox at "Cache LDAP queries" and tried to set `localhost` as the "Memcached Server to use". I also installed `memcached` on our Ubuntu machine using `apt install memcached`. `systemctl status memcached` tells me that the service is running. What am I missing?

Unfortunately there is no documentation about the "How to enable memcached". It is only mentioned that it can be used.

The property field "Memcached Server to use" always has the value `feature not available` which does not help me either. How can I make that feature available? Do I need an extra python package to install? Or an extra extension for Plone?

I am running Plone 5.2.1 with Python 3.6.8.

This issue is also about updating the documentation about caching to help other users. Thank you.

No file or test is named in the issue. Start by locating the existing caching documentation and the configuration behind the “Memcached Server to use” field, then verify the setup against Plone 5.2.1 and Python 3.6.8. Done means documenting the required packages or extensions, activation steps, and why the field may show “feature not available”.

Written by the indexing model from the issue text.