Do not expose OMERO_HOME in the doc (see #10603)

`OMERO_HOME` is an internal variable used in `bin/omero` and should not be set unless people really know what they are doing.

This PR replaces all occurrences of `OMERO_HOME` by either
- `OMERO_PREFIX` when pointing at a OMERO.server instance
- `OMERO_SOURCE_PREFIX` when pointing at the source code root directory