Add doc.bitcoinabc.org to the nav bar

Summary:
These docs are really only valuable to developers working on
ABC code directly, so Code Documentation was used to differentiate
between that and RPC Documentation which will come later.

Test Plan: Deploy locally and verify the link works

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6946