Fix broken links in sample repos with custom default branch (#9619)

The generated README.md for sample repositories hardcoded the
default branch as main. This meant that if a user specified a
custom default branch name, the image asset links would be
broken and tutorial commands would be incorrect.

This change introduces a new config field for the templater to
use the specified default branch and updates the README.md
template to use this new field. The existing screenshots still
use `main` as the default branch so various documentation lines
and image alt-texts have been updated to reflect this.

Closes #9157

Author: zubron