docs: Add Default option for branch-level squash settings
What does this MR do and why?
Documents the new "Default" option for branch-level squash settings introduced in !211017 (merged).
This option allows users to reset branch-level squash settings to inherit from project defaults, addressing the issue described in #575818.
Documentation changes
Updates the "Edit squash commits option" section in doc/user/project/repository/branches/branch_rules.md to:
- Document the new "Default" option
- Explain that it removes branch-level overrides and inherits from project settings
- Note that "Default" is not available for "All branches" rule (which defines project defaults)
Related to #575818