DEV_DOCS: Document the shared Cram setup scripts

[victorlin]

Description of proposed changes

Add a note on this now-consistent practice.

Related issue(s)

• https://github.com/nextstrain/augur/pull/1637
• https://github.com/nextstrain/augur/pull/1636#pullrequestreview-2323731468

Checklist

• [x] ~Automated checks pass~ Changes not checked
• [x] Check if you need to add a changelog message
• [x] Check if you need to add tests
• [x] Check if you need to update docs