[WEBSITE] Fetch the documentation content from the abc repo

Summary:
The _doc directory from the website contains files generated from the
ABC repo, and archived by version.

This diff adds a script that will generate the content, and will be used
by CI to populate the documentation at website build time. It also
allows for populating it locally, intended that the node software build
dependencies are installed. A cache mechanisme prevents from rebuilding
all the versions at each call.

This script is not run by default, so the website can still be tested
locally without the need for building anything.

Test Plan:

./scripts/fetch_documentation.sh

Check the _doc/<versions>/rpc and _doc/<versions>/man content is
populated. Now see to cache in action:

make doc
rake jekyll:doc

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7304