docs: add acceptance criteria audit template

[bonaventure001]

Context

I'm the assignee on #116. I noticed PR #127 (from a non-assignee) is
also open against this issue with a similar goal -- flagging that here
for transparency in case a maintainer wants to reconcile the two rather
than merge both.

Summary

Adds a per-criterion acceptance-criteria audit template, replacing the
flat checkbox list in Section 6 of the PR template with a table that
requires evidence per criterion.

Changes

• docs/acceptance-criteria-audit-template.md (new): audit table
  format, status values (Done / Partial / Deferred / N/A), rules for
  handling incomplete criteria, and a worked example
• PR template Section 6: checkbox list replaced with the audit table
• CONTRIBUTING.md and docs/pr-evidence-checklist.md: updated to
  reference the new template

Acceptance criteria mapping

• Acceptance criteria audit template is added -- docs/acceptance-criteria-audit-template.md
• Template includes implementation evidence -- Evidence column, with guidance on what counts as concrete evidence
• Template includes test evidence -- covered under the same Evidence column and the "Acceptable evidence" examples
• Template includes documentation impact -- covered via the worked example (retry-behaviour doc row)
• Incomplete criteria handling is explained -- dedicated "Handling incomplete criteria" section (Partial/Deferred rules, Relates to #N requirement)
• PR template or contributor docs reference the audit -- PR template, CONTRIBUTING.md, and pr-evidence-checklist.md all updated

Note for reviewers

Written independently, in this repo's existing definition-table style
(matching pr-evidence-checklist.md) rather than a generic table
format. Happy to adjust or step aside if a maintainer prefers #127.