Update references to default branch in docs

[emkll]

Description

Fixes #1122

Test Plan

Docs changes only

• URLs that were changed point to the correct files on the correct branch

[sssoleileraaa]

lgtm

notes about using HEAD instead of the branch name (thanks @maeve-fpf for the pro-tip):

• HEAD of a remote is defined as the "default branch" of the remote: https://git-scm.com/docs/git-remote#Documentation/git-remote.txt-emset-headem, so if you were doing self-hosted git somewhere, you would run git remote set-head origin main instead of going to the github UI, which as the docs say will set the symbolic-ref refs/remotes/origin/HEAD to refs/remotes/origin/master. This will only work if refs/remotes/origin/master already exists; if not it must be fetched first.