Improve generated docs site

[nicktelford]

We use the Maven Site Plugin to automatically generate a website with all of the project's reports and documentation when a release is made.

However, the site that's currently generated is difficult to navigate (especially to find the API docs, which is the most important thing) and riddled with bugs.

Ideally, we should find a way to make the Site plugin behave as we want. Alternatively, we should find another mechanism for generating an appropriate project site.

I'd really like to avoid having to maintain a site by hand, as it increases the chance of documentation becoming out-of-sync with the codebase.