Improving the experience for newcomers: Discoverability

[nickray]

As brought up in chat, I think the entry point for interested newcomers could and should be improved. These are some suggestions:

• [ ] Merge https://rust-embedded.org with https://docs.rust-embedded.org
• [ ] SEO (= discoverability) Work towards this https://rust-embedded.org becoming the landing page, that is among the first three results for googling "rust embedded" and similar (add keywords for major chip vendors and families etc.)
• [ ] Link to LP from https://github.com/rust-embedded
• [ ] Fix link in https://github.com/rust-embedded/docs (helps SEO too)
• [ ] Explain ecosystem around PAC/HAL/BSP/driver crates: https://github.com/rust-embedded/docs/issues/19
• [ ] Reach out to PACs to link back to homepage, and mention a HAL or BSP should be used instead of the PAC directly
• [ ] Pin relevant repos in https://github.com/rust-embedded to emphasize the important ones (e.g. cross does not belong in third place)

[phansch]

Would it perhaps make sense to merge the discovery book into the main book? I initially started out by googling 'rust embedded book' and didn't find out about the discovery book until I found a link to the bookshelf.