Documentation Updates

### Is your feature request related to a problem?

I was walking through the Gemara spec via the website and found it difficult to discover all of the information that I needed to use the spec effectively.

### Describe the solution you'd like

# Navigation

- I would like to be able to easily view all attributes and relationships of the spec on the website
- A translation from CUE -> OpenAPI -> Docusaurus+React Flow may be a potential solution for a more interactive experience
- I would like to be able to easily search the site
- I expected to be able to drill down into the different layers of [the model](https://gemara.openssf.org/model/) to be able to see the associated schema materials

# Missing Content

- The ADRs page doesn't contain a list of ADRs (maybe there haven't been any)
- When looking at the [schema](https://gemara.openssf.org/schema/) page, it isn't immediately obvious as to when/why I would use parts of the schema. User-oriented flows starting from "what problem does this flow solve" would be great

# Nits

- I would suggest placing a GitHub icon link at the top right of the page (I looked for it instinctively to file this ticket)
- If the [Roadmap](https://gemara.openssf.org/roadmap) linked back to the driving GitLab issues (or other artifacts) it would be useful for understanding the overall state of things and where I might contribute most effectively

### Describe alternatives you've considered

_No response_

### Success Criteria

_No response_

### Additional context

_No response_

Start by reviewing the existing model, schema, ADRs, and roadmap pages linked in the issue to map the current information architecture. The requested work spans navigation, search, user-oriented schema guidance, GitHub links, and roadmap references; done would require agreed scope and success criteria for the selected improvements.

Written by the indexing model from the issue text.