Set correct markdown extension for generated RPC docs

Summary:
Jekyll and other markdown processors do not correctly
recognize these files as markdown. Attempts to render as HTML
result in practically unusable pages. This patch fixes that
by setting the extension as expected.

Test Plan:

contrib/teamcity/build-configurations.py build-docs

Verify abc-ci-builds/build-docs/doc/rpc/* contains .md files.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7052