Remove language sub-route from RPC doc permalinks

Summary:
While it would be nice to support translated documentation, this is unlikely to
happen in the near future. Cleaning up the /en from the permalinks makes the URLs shorter,
easier to remember, and more consistent with https://www.bitcoinabc.org/doc/dev/ which does
not have translation support (and is unlikely to ever have it, given the development overhead
of maintaining translated comments).

The /en was retained in the file paths since it does not negatively impact the build system
and it gives a hint for where translated RPC documentation could go in the future. When that
day comes, then we can consider how those translated docs fit into our routing scheme.

Test Plan:
Build docs:

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

Copy the docs over to the bitcoinabc.org repo:

mkdir -p ~/projects/bitcoin-abc-website/doc/
cp -r abc-ci-builds/build-docs/doc/rpc/en/0.21.13 ~/projects/bitcoin-abc-website/doc/

Build and deploy locally:

make container
make run

Navigate to http://127.0.0.1:8080/doc/0.21.13/ in your browser.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7065