Update various references from master to main brach

[jnoordsij]

This is a new feature | improvement | bugfix | documentation fix.

Proposed solution

When checking some things when preparing another MR, I noticed some links were out-of-date.
This MR updates various references from master to main, given that seems to be the new default branch now.

Note: https://github.com/jgthms/bulma/blob/main/docs/documentation/start/responsiveness.html still has some master references too; these might need to be revised in some way too.

Tradeoffs

N/A

Testing Done

None.

Changelog updated?

No.