Fix doc errors

Hi,

this fixes various errors and warnings in the documentation. Only two are left, that I could not find a way to debug:

```
/tmp/buildd/bcfg2-1.4.0pre1/doc/server/encryption.txt:: ERROR: Anonymous hyperlink mismatch: 1 references but 0 targets.
See "backrefs" attribute for IDs.
<xmlschema>:2: WARNING: Block quote ends without a blank line; unexpected unindent.
```

The release announcements are not included anymore, but are linked as a toctree from the topic. This change is required because otherwise sphinx prints an error that the labels in the release notes are redefined (once in the included topic and a second time in the topic that includes the other topics).

Also the platform-specific quickstart notes get a better link title and are now a toctree to remove the warning, that some topics are not included in any toc.