🚀 Feature: Allow README.mds to start "Usage" section with an h2 other than "Usage"

### Feature Request Checklist

- [x] I have pulled the latest `main` branch of the repository.
- [x] I have [searched for related issues](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aissue) and found none that matched my issue.

### Overview

Right now, `blockREADME` assumes generated README.md files will have a `## Usage` h2:

https://github.com/JoshuaKGoldberg/create-typescript-app/blob/6bd31ee9e7f5ad0a3ad3964537f1047562e200ac/src/blocks/blockREADME.ts#L59-L63

That's true for most of my personal repos, but not all of them (see: https://github.com/JoshuaKGoldberg/prune-github-notifications/pull/201/files#r2028676849) - and certainly not everybody else's in the world. `## Usage` should not be assumed to exist.

Feature request: that `## Usage` heading should be part of `options.documentation.readme.usage`. Running `npx create-typescript-app` in transition mode should not add a `## Usage` heading if one does not exist already.

### Additional Info

🎁