docs(firestore-send-email): restructure the kit README to match the firestore-bigquery-export kit README #3315

firestore-send-email kit README. Priority: low. Do this last, after the other sub-issues of #3290 have landed.

What

Restructure kits/firestore-send-email/README.md so it is similar in structure and length to kits/firestore-bigquery-export/README.md. At kits @ 3ded546, the FBE README is 418 lines and 2,450 words; this one is 316 lines and 2,219 words.

Follow the FBE section order: Install, Required IAM, Usage, Deploy, Configuration, Multiple instances, Events, Provisioning, Failure handling, Migrating from the extension, Differences from the extension (including a Region subsection), API surface, Reporting bugs, License.

Sections this README is missing:

• Failure handling: what happens when a run fails, and how to find out.
• Migrating from the extension: firebase ext:migrate and firebase ext:export --mode functions.
• Region: a subsection under Differences. Fold DATABASE_REGION now decides where the function runs into it.
• Provisioning: only if it applies to this kit.

When

Wait until #3309 (which edits this README) has merged and #3300 (README fixes) is done. Then fold their content into the new structure rather than rewriting it.