Support version drop-down

[notmandatory]

There should be a drop down at top of book to select different versions of the BDK book, something like:

1.X - points to 1.0 then 1.1 then 1.2, etc.
2.X - points to 2.0, 2.1 etc.

Only needed after we release a 2.X version. For now nightly master branch could be generated to the main book site. After 2.X is out we freeze the 1.X book and add the drop-down and push nightly master to 2.X. Still need a way to update 1.X book .. maybe it's just a copy of the pages cloned to a sub-directory.