[RPC Docs] Use .html instead of .md extension for generated RPC docs

Summary:
This is a partial revert of D7052. Jekyll pre-renders all markdown to HTML
before rendering page layouts, so there is no way to access the raw file contents
without additional workarounds. Although this extension is wrong given the file contents,
this is clearly the path of least resistance for rendering these files without HTML
tags.

Test Plan:

build-configurations.py build-docs
cp -r abc-ci-builds/build-docs/rpc/docs/en/0.22.0 ~/projects/bitcoin-abc-website/_doc/
pushd ~/projects/bitcoin-abc
make serve

Navigate to http://localhost:4000/doc/0.22.0/rpc/blockchain/getblockchaininfo/ and verify
no HTML tags are present in the RPC doc content.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7206