This patch uses the new /doc/dev route instead of https://doc.bitcoinabc.org
everywhere in this repo. A placeholder is introduced to ensure the route is not
accidentally reused since the dev docs are built and hosted separately from deployments
of the main site.

make container
make run

Navigate, checking nav bar and the following links:

• http://127.0.0.1:8080/doc/ < should have a default index provided by nginx
• http://127.0.0.1:8080/doc/dev/ < should show the placeholder page