Add autoindexing for /doc route

Summary:
This is a temporary measure so that the RPC docs are easy to navigate until
proper indexing pages are created. This allows us to easily test and deploy RPC
documentation on various routes under /doc/ without needing to update configs.
A simple redeploy will suffice.

Test Plan:

cp ~/projects/bitcoin-abc/abc-ci-builds/build-docs/doc .
make container
make run

Visit http://127.0.0.1:8080/doc/ in a browser and navigate.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7057