Rename `source` branch to `main`

[dimaMachina]

Description

main is a common pattern for the principal branch

Motivation

Collaboration

Additional Context

[saihaj]

@benjie @leebyron any objections on this one?

[benjie]

None; assuming it doesn’t break anything. I think it used to be because main was where the GitHub pages site deployed from? Anyway go ahead; we can always change it back if need be.