Make the doc directory a collection

Summary:
This makes it easy to templatize indices based on the
files under the doc collection.

Test Plan:

make container
make run

Navigate to http://localhost:8080/doc/ and ensure the nginx indexing is still working as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7127