Add Docs for Union Types via the [@unboxed] feature

[ryyppy]

Create a standalone page "Union Types", which basically translates the previous BS blog posts into a more formal documentation page:

• Union Types in BS
• BS 7.2 Holiday release

This includes adding the [@unboxed] decorator to the decorators overview with a link to the actual doc (an example would be too big to just inline it).

\cc @fhammerschmidt

[ryyppy]

Since I have been investing quite some time in reiterating Tagged Unions w/ Variants, and since there was a lot of feedback coming from Cristiano's side regarding the unboxed feature (and its blog post), we probably want to split that topic up into more practical examples, independent from the "interop" sidebar category and really think about useful use-cases