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

Fixes the broken main.tf link reported in the tasks-docker template
README.

The relative link `./main.tf` was returning a 404 error on the registry
website. This PR replaces it with the correct GitHub URL pointing to the
main.tf file.

**Changes:**
- Updated the main.tf link in
`/registry/coder-labs/templates/tasks-docker/README.md`
- Changed from `[main.tf](./main.tf)` to
`[main.tf](https://github.com/coder/registry/blob/main/registry/coder-labs/templates/tasks-docker/main.tf)`

Tested: Verified the new link works correctly.

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Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>