Add 'Request a Repo' feature to the repos homepage

[jonathanpopham]

Context

From internal discussion: the architecture docs for repos like volt aren't easily discoverable from the main repos page at repos.supermodeltools.com. Users searching for a specific repo's arch docs can't find them unless they have a direct link.

Request

Add a way for anyone to request a public repo be added to the main repos homepage (supermodeltools/supermodeltools.github.io).

Proposed Implementation

1. GitHub Issue Template — Add an issue template to supermodeltools.github.io with a structured form for requesting a repo (repo URL, category, description, etc.)
2. "Request a Repo" button — Add a visible link/button on the homepage (repos.supermodeltools.com) that directs users to open a new issue using the template

This keeps the process lightweight: anyone can request, maintainers review and approve via the existing repos.yaml + setup-community-repos.sh pipeline.

Implementation Repo

The actual changes will be made in supermodeltools/supermodeltools.github.io since that's where the homepage and repo registry live.