docs: update CI/CD GitHub Actions guide to use environment-scoped secrets

[Copilot]

REQUIRED Corresponding PR: adobe/generator-aio-app#298 that needs a release

The generated CI/CD workflows now use GitHub Actions environments (stage, production) to scope secrets natively, replacing the old convention of duplicating secrets under _STAGE/_PROD suffixed names in the repository secret store.

Documentation updates

• Step 4 — instructions now point to Settings > Environments to create a stage environment and add secrets there; removes the outdated note saying environment secrets were unsupported
• Step 4 table — all 14 secret names drop the _STAGE suffix (e.g. CLIENTID_STAGE → CLIENTID)
• Step 5 — custom secret example updated (MY_CUSTOM_SECRET_STAGE → MY_CUSTOM_SECRET in the stage environment)
• Step 6 — instructs creating a production environment with the same secret names (environment scoping keeps values separate); removes the instruction to use _PROD suffix
• Step 6 table — all 14 secret names drop the _PROD suffix
• Step 7 — updated to set the environment field in the workflow YAML rather than inventing a new suffix

Before / After

# Before — repository-level secrets, suffix-differentiated
deploy:
  env:
    CLIENTID: ${{ secrets.CLIENTID_PROD }}

# After — environment-scoped secrets, no suffix needed
deploy:
  environment: production
  env:
    CLIENTID: ${{ secrets.CLIENTID }}