add features repo to index

[rcleveng]

What type of PR is this?

• [X] Add a new dev container collection
• [ ] Update to an existing dev container collection
• [ ] Documentation/spec update
• [ ] Other containers.dev site update (UX, layout, etc)

Related Issues

• Related Issue #
• Closes https://github.com/rcleveng/devcontainer-features/issues/4

Description

Adds a new devcontainer repo to the index

Collection checklist

If your PR contributes a new collection, please utilize this checklist:

• [X] Collection name
• [X] Maintainer name
• [X] Maintainer contact link (i.e. link to a GitHub repo, email)
• [X] Repository URL
• [X] OCI Reference
• [X] I acknowledge that this collection provides new functionality, distinct from the existing collections part of this index.