OPENDJ-2657 Activate docs profile in default build

Reference documentation generation has been partially extracted
into the `opendj-man-pages` git repository.

Rather than extract the remaining generation steps,
this patch simply activates the docs profile by default.

Advantages:

*  Minimal changes to the code.
*  Regressions that break doc generation show up immediately.

Disadvantages:

*  Slightly longer build (~2 seconds)
*  Build produces separate doc artifacts

Jenkins jobs might need to enable `Deploy maven artifacts`
with `Include Patterns` set to deploy the doc artifacts:

```
**/opendj-server-legacy*generated-doc-sources*.jar **/opendj-server-legacy*config-ref*.jar
```