update stale timing and add ai policy (#26465)

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## Description

Reduce time to stale for PRs.
Link to the Docker open source AI contribution policy in our
contribution guidelines.

## Related issues or tickets

https://docker.slack.com/archives/C039ZM6GH4Z/p1793044325677639

## Reviews

<!-- Notes for reviewers here -->
<!-- List applicable reviews (optionally @tag reviewers) -->

- [ ] Editorial review

Signed-off-by: Craig Osterhout <craig.osterhout@docker.com>

.github/workflows/stale.yml

@@ -198,7 +198,7 @@ jobs:
           # days-before-pr-close, also update the hardcoded values in the
           # stale-issue-message and stale-pr-message above to match.
           days-before-issue-stale: 180  # 6 months
-          days-before-pr-stale: 180     # 6 months
+          days-before-pr-stale: 90      # 3 months
           days-before-issue-close: 14   # 2 weeks after stale
           days-before-pr-close: 30      # 1 month after stale
           

CONTRIBUTING.md

@@ -9,6 +9,8 @@ Before contributing, review our documentation standards:
   guidelines
 - [COMPONENTS.md](COMPONENTS.md) - How to use Hugo shortcodes, front matter,
   and components
+- [AI policy](https://github.com/moby/.github/blob/main/AI_POLICY.md): Rules
+  for using AI tools when you contribute
 
 The following guidelines describe how to contribute to the
 Docker documentation at <https://docs.docker.com/>, and how to get started.