usage: advanced: improve documentation of manual docker image and autopilot firmware sources

This is already partially covered by the link to the BlueOS CI from the "Manual upload" UI card in the BlueOS version chooser, but we should also include that and other relevant links/mentions in the docs.

In particular:

- `arm-v7` / `arm64-v8` images from [the GitHub releases](https://github.com/bluerobotics/BlueOS/releases)
- [CI Artifacts](https://github.com/bluerobotics/BlueOS-docker/actions/workflows/test-and-deploy.yml) for testing pull requests and the `master` branch
- custom/local images generated using `docker save`

The autopilot firmware can be:
- downloaded from [the firmware site](https://firmware.ardupilot.org)
- built locally (from source) [with waf](https://ardupilot.org/dev/docs/building-the-code.html)

Question raised in [this forum post](https://discuss.bluerobotics.com/t/how-to-download-blueos-core-for-offline-update/17011), whereby the user did not know where to find available image downloads.