Convert docs site to Jaspr

### Problem to solve

It would be great to have our docs site in [Jaspr](https://jaspr.site/) to have even more of our code-base in Dart (and avoid weird package update problems from other ecosystems etc).

> Jaspr is a free and open source framework for building websites in Dart.
Create fast and dynamic web experiences with ease in a familiar language and ecosystem.

### Proposal

We would need the following from Jaspr:

1. Docs written in markdown
2. Flutter examples embedded on the page
3. Mermaid diagram rendering
4. Syntax highlighting for code

According to @schultek 1, 2 and 4 already exists and 3 should be possible to create.

### More information

- Currently we are using [Sphinx](https://www.sphinx-doc.org)
- The root of our docs live here: https://github.com/flame-engine/flame/tree/main/doc
- The docs are built and published from this repository: https://github.com/flame-engine/flame-docs-site/

### Other

- [ ] Are you interested in working on a PR for this?