chore(ci): skip docs-only workflow runs

[Teakowa]

Summary

• skip CI when a push or pull request changes only Markdown files or docs/**
• keep full Rust and cross-platform validation for any other change
• avoid per-job path classifiers and keep the workflow easy to maintain

Note

GitHub leaves path-filtered required workflows pending. If this CI workflow is later configured as a required check, the ruleset should account for that before relying on docs-only skips.

Scope

CI orchestration only; no OPY semantics changed.