Register the repo with Context7 for documentation retrieval

### Feature Request / Improvement

Register the repository with Context7 so assistants can retrieve current XTable documentation instead of relying on stale training data.

`apache/hudi` does this with a two-line file at their repository root:

```json
{
"url": "https://context7.com/apache/hudi",
"public_key": "..."
}
```

## Why

XTable's configuration surface and format support change every release. Answers people get from assistants today are drawn from whatever was in training data, which means old config keys and formats we no longer recommend. Doc retrieval fixes that without any code.

## Scope

- Add a `context7.json` at the repository root, pointing at an XTable entry.
- Confirm there is nothing in it that conflicts with ASF policy, since it references a third-party service and carries a public key.
- Check whether the file needs an ASF license header, or an exclusion in the RAT configuration if the format does not permit comments.

Small and independent of the rest of this umbrella. Reasonable as a first contribution.

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

Add context7.json at the repository root, using the referenced apache/hudi example as a starting point for the XTable entry. First check ASF policy implications for the third-party service and whether the RAT configuration needs an exclusion because JSON cannot contain a license header. Done means the file points to XTable, includes the required public key, and passes the repository’s policy and RAT checks.

Written by the indexing model from the issue text.