Add link to bcha page to main nav

Summary:
As requested adding a link to the new page to the main nav.
Listed it first but open to other placements

Test Plan: Make sure it works on live site

Reviewers: jasonbcox, #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D8489