Define Downstream Rollout and Maintenance Guidance

[Copilot]

Closes#5

Downstream maintainers had no single reference covering what to do after templating a repo, which files are mandatory vs removable, or how to track template evolution.

Changes

docs/DOWNSTREAM_GUIDE.md (new)

Five-section guide covering:

• Minimum post-template setup — ordered steps required before the repo is usable
• Required customization points — table of every file containing placeholders or instructional text that must be replaced
• Mandatory items to retain — community health and governance files that must stay present
• Optional items — what can be removed and under what conditions; explicitly scopes stack-specific automation and compliance tooling as out-of-default (informed by discussions Which Optional Extensions Should Stay Outside the Default Template? #7 and What Should Remain Mandatory in Downstream Repositories? #8)
• Template evolution — how downstream repos track changes and what compatibility the source template guarantees

TEMPLATE_CHECKLIST.md

Added a final-review item directing maintainers to confirm required customization points against the new guide before deleting the checklist.

docs/README.md

Linked DOWNSTREAM_GUIDE.md at the top of the template administration resources list.