[WEB] Add a link to the new Bitcoin ABC Blog

Summary:
This adds a new link to the new Bitcoin ABC blog on the right of the current menu.

Test Plan:
Look at the result:

bundle exec jekyll serve

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6637