Migrate website/documentation into main git repo

[abrokenjester]

We currently have a separate git repository for the website and documentation. This makes it somewhat difficult to keep track of whether or not new features have been properly documented, which issues changes to the website/documentation belong to, etc.

I'd like to propose that we move the rdf4j-doc contents into the main git repository, so that code changes and accompanying documentation changes can be solved in the same pull request if we want. It will also make updating the published javadoc easier.

[abrokenjester]

One thing I haven't fully figured out yet is how we handle (continuous) deployment of the website in a single repo. Currently, the website is deployed straight from the master branch (the website host server runs a git pull and a site build on a regular schedule), but if we start handling things in a single repo, we don't (always) want documentation updates for things that haven't yet been released.

Though perhaps, if master is always used for bug fixes only (including "bug fixes" in the website/documentation) this isn't a big deal.

[abrokenjester]

Scheduled to start on this after the 3.3.0 release, to minimize the number of parallel branches I will have to transport.

[abrokenjester]

Main branch merged. Website server reconfigured to use the main rdf4j repo to generate the site. We're migrated.