executor: Finish up usable docs

Created by: efritz

New docs to create:

• #27518 (closed)
• #27437 (closed)
• #27438 (closed)
• #27439 (closed)

Edits to AWS/GCP Terraform modules:

• #27440 (closed)

Edits to Deploying Sourcegraph executors:

• #27441 (closed)
• #27442 (closed)
• #27443 (closed)
• #27444

Dry-run notes

• #27864 (closed)
• #27865 (closed)
• #27866 (closed)
• #27867 (closed)
• #27868 (closed)
• #27869 (closed)
• #27870
• #27871 (closed)
• #27877 (closed)
• #27878 (closed)
• #27887