Consistent navigation between independent website systems

[straight-shoota]

The core of the Crystal website is in this repository and everything in it shares a common navigation system. But there are also some external components that serve as extensions to the website.
Most notably, the Crystal book. But also the forums and API docs. #286 lists a number of more external resources with less direct integration.

We should improve the interconnectivity of the different resources to improve usability. It shouldn't matter which technical solution we use for different parts of the website. Users should be able to navigate between them as seamlessly as possible. This relates strongly to #284 but isn't exactly identical. Consistent navigation may affect other elements than the main navigation menu, for example.

The main focus for that are the book and forum. They're both linked prominently from the website's main navigation, but there are only limited backlinks between them and the other parts of the website.
We recently added a website link to the forum's top navigation. From the book, there is currently no direct link to the forum or other parts of the website (only deep links to some pages like the installation instructions).
Most of the other external resources are integrated into platforms which we have no control over. Maybe the playground could be integrated, though.

Ideally, we would have a common navigation menu that is identical or at least very similar across all different parts. This may cause some conflicts with the internal navigation of each individual part. We might also be limited by the amount of customization we have over the individual components (especially the forum which is hosted externally). But we should definitely have options. As soon as we trim the overly long forum title down, there's more space for nav items:

[straight-shoota]

Related: #51