OMERO.server properties: documentation template

In preparation of the review of multiple configuration sections for 5.0.3 (search, memory...), this PR proposes to move each configuration topic to a standalone page for clarity. An introduction section for the advanced properties is kept at the beginning explaining how to modify the config using the CLI, referring to `etc/omero.properties` and listing the following topics.

Additional changes that could be considered/implemented as part of this PR:
- unify all the configuration topics (e.g. LDAP ) to use glossaries
- always put the list of properties at the beginning of each advanced configuration page with a generic title (e.g. `Configuration properties`)

Another breaking change could be envisioned: as the number of sections will grow, this could be moved to another level of hierarchy (like we did for the basic UNIX/Windows installation) with an index page, e.g.

```
omero/sysadmins/configuration/index.html
omero/sysadmins/configuration/memory.html
omero/sysadmins/configuration/search.html
omero/sysadmins/configuration/ldap.html
```
The caveat of this approach is that only the index page would  be visible from the main sysadmin TOC. On the flip side, this would generate a simple scheme for adding server configuration pages.
Ahh, yes, that makes sense, thanks. (Maybe we need a `bin/omero config get` variant that includes all the defaults from the `etc/omero.properties` that is being currently applied!)