Add heading to release notes template

Summary:
When rendered as a page, release notes are a little odd as there is no title.
(see https://www.bitcoinabc.org/doc/release-notes/release-notes-0.22.2.html for example)

This patch adds a heading to the release notes template and updates all past ABC
release notes using

cd doc/release-notes
grep -o "ABC version [0-9.]*" ./*.md | cut -d' ' -f3 | \
  xargs -n1 -I{} sed -i '1s/^/# Bitcoin ABC {} Release Notes\n\n/' ./release-notes-{}.md

Test Plan:

cd ~/projects/bitcoin-abc-website
./scripts/fetch_markdown_files.sh ~/projects/bitcoin-abc
make serve

Navigate to http://localhost:4000/doc/ and continue navigating to some release notes pages.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7544