Update headmatter to be consistent and parseable for stac-api-extensions.github.io/

[philvarner]

For example, this is from the Aggregation extension:

- **Title:** Aggregation
- **OpenAPI specification:** TBD
- **Conformance Classes:**
  - <https://api.stacspec.org/v0.2.0/aggregation>
- **Scope:** STAC API - Core
- **[Extension Maturity Classification](https://github.com/radiantearth/stac-api-spec/tree/main/README.md#maturity-classification):** Proposal
- **Dependencies:**
  - [STAC API - Core](https://github.com/radiantearth/stac-api-spec/tree/v1.0.0-rc.2/core)
- **Owner**: @philvarner

[m-mohr]

Done