Add version switcher to the menu

[badluck13]

Add documentation version switcher to the menu, something similar the other frameworks already do, below are examples from bootstrap and angular documentation.

I also noticed there is v4 link in v5 dev branch but there is no link from v4 to latest, also there is no exact version shown in documentation that could be used as a good reference - v4.6.6 or v4.6.x would be better then just v4.

[kamilmysliwiec]

Thanks @badluck13, definitely has to be done in the near future.

[badluck13]

@kamilmysliwiec will see if I get some time to contribute a bit, I'm quite enthusiastic for nestjs future