Write up GitHub Pages migration

[cmbuckley]

See #559, cmbuckley/.github#4, cmbuckley/showcase-skeleton#1

[cmbuckley]

Other notes:

• edit link - reads the branch from the Pages API
• workflow/repo dispatch events must be on default branch
• means code and doc branch can’t be completely isolated
• could this be consolidated in future? Just build docs from readme + docs folder
• steps to update default branch, update branch protection etc
• remote theme - no _plugins
• all because of Jekyll 4 etc