Sort docs by version, with latest at the top

Summary:
This patch fixes two issues:

1. The latest version is not the first thing users see on the docs index.
2. The sorting is broken. This issue will not become evident until 0.22.10, but it will be very broken once we get there without this patch.

Test Plan:
In order to verify the sorting is correct, you will need to generate some bogus
docs by copying the existing ones and replacing version numbers, for example:

cp -r 0.22.1 0.22.10
find "0.22.10" -type f -print0 | xargs -0 sed -i 's/0.22.1/0.22.10/g'

make serve

Navigate to http://127.0.0.1:4000/doc/
Verify the order is correct (latest to oldest) and sorting is correct (0.22.10 comes after 0.22.1).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7456