[WEB] Add a Developers menu with Contributing and Code coverage

Summary:
This adds a new link to the last code coverage report and put it under a
Developers menu next to Contributing.

Test Plan:
Look at the result:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6636