Changes are:
- Remove unix- and stm32-specific sections (move unix to its own
  README.md), stm32 was duplicated.
- Add links to GitHub Discussions and Discord.
- Update information about the project.
- Add a getting started section.
- Explain `make submodules`.

Signed-off-by: Jim Mussared <[email protected]>
Signed-off-by: Damien George <[email protected]>