Missing cross-references for OMERO properties

[sbesson]

Noticed while working on #2433, the cross-references for the OMERO properties are currently broken. Looking at the historical releases, this regression seems to have introduced during the migration to readthedocs.
For instance, compare the omero.ldap.base markup in https://docs.openmicroscopy.org/omero/5.6.4/sysadmins/server-ldap.html vs https://omero.readthedocs.io/en/v5.6.5/sysadmins/server-ldap.html.

The relevant logic is in

omero-documentation/omero/conf.py

Lines 407 to 411 in 72fa9fe

	app.add_crossref_type(
	directivename = "property",
	rolename = "property",
	indextemplate = "%s",
	)

There are a few factors that might have caused this regression but the two obvious ones coming to mind are:

1- change in newer Sphinx major version (5.6.4 was built with Sphinx 3 vs Sphinx 5 for 5.6.5)
2- self-hosted vs readthedocs infrastructure.