Clarify versioning/support policy for the various OMERO components

[sbesson]

See the conversation starting at ome/omero-test-infra#63 (comment)

It would be useful to have some official page explaining:

1- the current versioning policy for individual components (both Java, Python). My assumption is semver everywhere except maybe for the OMERO.server bundle. Or would we release backwards compatible API additions as OMERO.server 5.7 ?
2- the compatibility between components. Minimally, I think we can state that OMERO.server 5.6.x should be compatible with any OMERO.web/OMERO.py 5.y. Related to the question above, would we also say that any OMERO.server 5.x (with x>=5) should be compatible with OMERO.web/OMERO.py 5.y ?

[sbesson]

Closing in favor of #2311