Skip e2e and unit tests for docs-only changes

[smoparth]

Doc-only PRs (e.g., proposals in docs/) trigger the full CI matrix including
e2e tests, unit tests, and CodeQL analysis. This wastes CI time on checks that
cannot be affected by documentation changes.

For example, PR #1225 (a single markdown file in docs/proposals/) ran 30+ e2e
jobs across multiple Python versions and platforms, taking over 14 minutes on
the longest job.

Proposed fix

Add paths-ignore for docs/** to the following workflow files:

• .github/workflows/test.yaml (unit and e2e tests)
• .github/workflows/codeql.yaml (CodeQL analysis)
  Leave .github/workflows/check.yaml unchanged since it runs the Sphinx docs
  build, which validates that proposal files render correctly.

Context

Discussed with maintainers on Slack. Agreed that:

• Python tests (unit, e2e) don't need to run for doc-only changes
• CodeQL static analysis doesn't need to run for doc-only changes
• Sphinx build (in check.yaml) should still run since proposals are part of the
  official docs