Rename default branch to main

[Kobzol]

I kept the ?version=master link in the docs page to keep backwards compatibility.

See also rust-lang/team#2056

[Kobzol]

I kept the old ?version=master link in docs to preserve backwards compatibility there.

[BenjaminBrienen]

I kept the old ?version=master link in docs to preserve backwards compatibility there.

I see one changed in Changelog.md. Is that unintentional, then?

[Kobzol]

To clarify, both master and main should work in the URL now, master is just kept for backwards compatibility. So changing it in the changelog shouldn't matter.