Archive the GSA/data-strategy repo

## Summary

After the redirect is live and confirmed, archive the `GSA/data-strategy` GitHub repository.

## Background

With strategy.data.gov redirecting to resources.data.gov, the `GSA/data-strategy` repo no longer needs active maintenance. Archiving it prevents accidental changes and signals to contributors that the project is closed.

## Tasks

- [ ] Update `README.md` to note that the repo is archived, that strategy.data.gov now redirects to resources.data.gov, and include the date
- [ ] Archive the repository via Settings > Archive this repository

## Acceptance criteria

- [ ] Repo is marked archived on GitHub
- [ ] README updated with archival note and redirect date

## Depends on

Ticket 6