No documentation on q search

[archaeogeek]

Although the q search is apparently still using the old api and doesn't appear in the swagger documentation, it is very useful and it would be helpful if it was documented.

Note this is in progress, based on the information supplied by @josegar74 in this thread on the user mailing list: https://sourceforge.net/p/geonetwork/mailman/message/36627750/

I propose to include it in the api section of the website as an option in this page: https://geonetwork-opensource.org/manuals/trunk/en/api/index.html