adding web settings parser

This PR should be tested with OMERO.web settings improvement in #3019. It adds OMERO.web setting parser to extend glossary by the ```dist/bin/omero config parse --rst```

After server section you should see:

```
OMERO.web properties
====================

.. setting:: FORCE_SCRIPT_NAME 

FORCE_SCRIPT_NAME
-----------------

``bin/omero config set omero.web.force_script_name``

Default: ``None`` 

Description: Used as the value of the SCRIPT_NAME environment variable in any HTTP request.

and more ...
```

Additional documentation changes are in https://github.com/openmicroscopy/ome-documentation/pull/983