docs: require signed commits

[Copilot]

Description

Contributor docs only mentioned the DCO Signed-off-by line and never told contributors to cryptographically sign commits. Adds the missing guidance, written once and linked from related docs.

• first-commit-06-creating-a-pr/index.md: New "Signing your commits" section — calls out that signing is separate from Signed-off-by, lists the three supported formats (GPG / SSH / S/MIME), links to the four relevant GitHub docs, and shows the git config --global commit.gpgsign true one-liner.
• contributing-pull-requests/README.md: "Signing your commits" section is a one-line pointer to the canonical section above — no duplicated prose.

Type of change

• This pull request is a minor refactor, code cleanup, test improvement, or other maintenance task and doesn't change the functionality of Radius (issue link optional).

Contributor checklist

Please verify that the PR meets the following requirements, where applicable:

• An overview of proposed schema changes is included in a linked GitHub issue.
  • Yes
  • Not applicable
• A design document is added or updated under eng/design-notes/ in this repository, if new APIs are being introduced.
  • Yes
  • Not applicable
• The design document has been reviewed and approved by Radius maintainers/approvers.
  • Yes
  • Not applicable
• A PR for resource-types-contrib is created, if resource types or recipes are affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for dashboard is created, if the Radius Dashboard is affected by the changes in this PR.
  • Yes
  • Not applicable
• A PR for the documentation repository is created, if the changes in this PR affect the documentation or any user facing updates are made.
  • Yes
  • Not applicable

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[Copilot]

Pull request overview

Updates the contributor documentation to explicitly require cryptographic commit signing (separate from DCO sign-off), centralizing the guidance in the “first commit” guide and linking to it from the PR guidelines to avoid duplicated instructions.

Changes:

• Add a new “Signing your commits” section to the first-commit PR-creation guide, including supported signature types and links to GitHub’s official docs.
• Add a short pointer section in the pull request contributing guide that links to the canonical signing instructions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/contributing/contributing-pull-requests/README.md	Adds a “Signing your commits” section that links to the canonical guidance.
docs/contributing/contributing-code/contributing-code-first-commit/first-commit-06-creating-a-pr/index.md	Adds the canonical commit-signing guidance, references, and a git config one-liner.