[WEBSITE] Add a link to go back from documentation to version selection

Summary:
This diff adds a link on top of the left menu to go back to version
selection.

Test Plan: Build the website and navigate through the documentation.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7333