development: extensions: walk through development process and examples

- [x] Fill out [the development process section](https://github.com/bluerobotics/ardusub-zola/blob/BlueOS-1.1/extensions/index.md#development-process)
- [ ] Refine and properly integrate [development-process.mermaid](https://github.com/bluerobotics/ardusub-zola/blob/BlueOS-1.1/extensions/development-process.mermaid)
- [ ] Include a section on optimisation approaches, mentioning
- Docker's re-use of shared image bases (e.g. encourage developers to use the same bases as other extensions where possible)
- Tools like [dive](https://github.com/wagoodman/dive) (for image introspection)
- [x] Complete the [accessing and (persistently) modifying an installed extension](https://github.com/bluerobotics/ardusub-zola/blob/BlueOS-devel-temp/development/extensions/index.md?plain=1#L464-L494) section
- [ ] Cover [cross-extension communications](https://discuss.bluerobotics.com/t/how-to-communicate-between-blueos-extensions/16987/3)
- [ ] Cover [offline/manual uploads/installs](https://github.com/bluerobotics/BlueOS/issues/1614)