Consider renaming default branch to main

[tclune]

Just a socially conscious recommendation.

https://github.com/github/renaming

[certik]

Thanks. We considered that and renamed other repositories. But with this one we need to do quite a few fixes:

https://github.com/fortran-lang/fortran-lang.org/search?q=master

However if you send us PRs, it can happen faster. :)

[awvwgk]

Duplicate of #325

[awvwgk]

This repository will be migrated to https://github.com/fortran-lang/webpage soon (see #415), where we use main as name for the default branch.