Improving the experience for newcomers: Content

[nickray]

There is a lot of content, with #18 it should become more discoverable.

The main missing point (brought up recently in chat) is an explanation of how the ecosystem is organised around PAC/HAL/BSP/driver crates, the various stm32-rs/nrf-rs/lpc-rs etc. organizations....

The book and discovery book are great, but they're written in a pedagogical, semi-magical style hiding the details of how to go around extending or writing one's own.

(I can try and add a PR addressing this, however I don't feel I have the authority to "prescribe" what in my understanding is more of an implicit status quo).