doc/src/backport_policy: add maintainer changes to backport criteria

[trueNAHO]

Add maintainer changes to the backport criteria to remove ambiguity
about whether they are included in the existing CI condition.

Maintainer changes should be backported to keep maintainer notifications
in sync between the unstable and stable branches.

Should we really explicitly mention this or is the CI implication enough?

---

• I certify that I have the right to submit this contribution under the MIT license
• Commit messages adhere to Stylix commit conventions
• Theming changes adhere to the Stylix style guide
• Changes have been tested locally
• Changes have been tested in testbeds
• Each commit in this PR is suitable for backport

[trueNAHO]

Alternatively, what about appending to the constraint clarifications:

Suggested change

	fixes, bug fixes, CI changes, and maintainer changes are backported, while new
	features, modules, and theme improvements are not backported. Upstream changes
	causing theming issues are considered a bug.
	fixes, bug fixes, and CI changes are backported, while new features, modules,
	and theme improvements are not backported. Upstream changes causing theming
	issues are considered a bug and maintainer changes are considered as CI changes.

IMHO, this is cleaner, if we want this clarification.