Split release page into BCHA and BCHN

Summary:
This assumes same version numbers and binary names exist
for every release. Obsolete releases have been removed to simplify
code maintenance. They can still be found via the Archived Releases
link at the bottom of the page. New releases will continue to display
in the table.

Test Plan:
Spoof release notes for soon-to-be-released 0.22.6 and serve:

scripts/fetch_documentation.sh
cp abc_md_docs/doc/release-notes/release-notes-0.22.5.md abc_md_docs/doc/release-notes/release-notes-0.22.6.md
cp abc_md_docs/doc/release-notes/release-notes-0.22.5.page.md abc_md_docs/doc/release-notes/release-notes-0.22.6.page.md
make serve

Navigate to http://127.0.0.1:4000/releases/

Alter the version number in {% if release.name == "0.22.6" %} to test
the Past Releases table.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8290