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 <jim.mussared@gmail.com>
Signed-off-by: Damien George <damien@micropython.org>
