Rename default branch from master to main

[jt-nti]

It looks like many zowe repos already use main instead of master so it would be great to update the docs-site repo as well.

For background, see
https://www.aswf.io/inclusive-language-guide/

[anaxceron]

Will be taking this up at a future TSC meeting for discussion.

[jt-nti]

Thanks @anaxceron

[anaxceron]

We agree that this is something that should be addressed, but as the doc squad, we do not have the bandwidth or the technical knowledge for this. If you are able and would like to take this on, we'd appreciate the help.

[jt-nti]

I think it would involve someone from OMP or the Linux Foundation with the right permissions but I'm very happy to investigate.