Add support for experimental properties markup in omero.properties

As a follow-up of #3078, this PR adds support for a per-property `EXPERIMENTAL` markup allowing to hide individual properties from the auto-generated Sphinx documentation page.
Exposition a configuration property in production mode should be as easy as removing the `# EXPERIMENTAL` line in `etc/omero.properties`

To test this PR, check the auto-generated branch still does not include the `omero.security.ignore_case` property or its description.

/cc @bpindelski