Rename default branch to 'main'

[inexorabletash]

As part of adopting neutral language in W3C specifications, the default branch for this repo should get renamed to 'main'

[anssiko]

Done per https://github.com/github/renaming#renaming-existing-branches

Let me know if something broke. You may want to s/master/main in .mds but meanwhile redirects should work.

To update your local environments:

git branch -m master main
git fetch origin
git branch -u origin/main main

[anssiko]

(Note-to-self: next time when renaming, disable and then re-enable GitHub Pages after the rename. Otherwise, GitHub Pages will give 404.)