add MetaSearch documentation

cc @rduivenvoorde 

This PR adds MetaSearch documentation as part of moving the plugin to QGIS proper (see https://github.com/geopython/MetaSearch/issues/62).

The docs are already translated in Transifex see:

- https://www.transifex.com/projects/p/MetaSearch/resource/docs_howto/
- https://www.transifex.com/projects/p/MetaSearch/resource/docs_index/

..and can be copy/pasted over as part of translation within core QGIS.  There are some slight resulting changes (one .rst file, some text changes).

Note that there is a local Polish translation of MetaSearch with QGIS, but I'm not sure that the docs are included (just the code/UI iirc)