feat: version switcher

[mischah]

Hej, here’s the implementation of option 3 of the version switcher as proposed in #5.

Must say that I prefer it over the implementation of option 1 in #9

Pros:

• You can switch from older version to newer
• Less changes in publish.js
  • Makes it easier to integrate future changes from JSDocs default template

Cons:

• We are loosing the custom page titles from the docs
  • Cause we only see the output from the <title> Element of the page holding that iFrame
• We are loosing bookmarkability and deeplinking to subpages

Closes #5