Show PixiJS version in docs header

[StanFlint]

Description of the problem

I often find myself googling the PixiJS documentation for something like Sprite or Application. The problem is these links are often for random versions. Whilst I am reading them, I have no idea which version I am reading (I have to check the URL).

Suggestion to fix the problem

Add a select menu in the header for the PixiJS version. This solves two problems:

• it is easy to discover which version of the docs you are viewing, and catch yourself if you are mistakenly looking at the wrong version!
• allows users to quickly navigate to their desired PixiJS version once they discover they are looking at the wrong one

Before	After (mockup)