Add a generic back button to markdown-generated pages

Summary:
Pages generated from markdown in the ABC repo have a worse
navigation experience than other pages. Compare how
https://www.bitcoinabc.org/CONTRIBUTING.html
is missing any sort of back button like the RPC docs have:
https://www.bitcoinabc.org/doc/0.22.2/rpc/control/help/

This becomes especially apparent when these patches are applied: D7457 or D7438

While it's difficult to provide context-aware back buttons for these pages, we
can add a simple Back button that mimics the browser back button. This helps
make the navigation more consistent between pages.

Test Plan:

arc patch D7457   # to best demonstrate this patch
make serve

Navigate to http://127.0.0.1:4000/doc/ and click one of the release notes, then click Back

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7461