Closes#194 alongside #325
## Description
This PR adds the missing base layer of validation for all Coder template
README files, ensuring that they all follow a consistent structure when
processed by the Registry website's build step. It also updates a few
README files to match the new standards.
## Type of Change
- [ ] New module
- [x] Bug fix
- [ ] Feature/enhancement
- [ ] Documentation
- [x] Other
## Testing & Validation
- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun run fmt`)
- [x] Changes tested locally
