Improve community docs

[andreeaflorescu]

Summary of the PR

Extend the existing documentation to reduce the tribal knowledge. There are still some things that would need to be improved but I would handle them as part of a separate PR.

Requirements

Before submitting your PR, please make sure you addressed the following requirements:

• [x] All commits in this PR are signed (with git commit -s), and the commit message has max 60 characters for the summary and max 75 characters for each description line.
• [x] All added/changed functionality has a corresponding unit/integration test.
• [x] All added/changed public-facing functionality has entries in the "Upcoming Release" section of CHANGELOG.md (if no such section exists, please create one).
• [x] Any newly added unsafe code is properly documented.

[andreeaflorescu]

I'd still like to add some details about how to update the Rust version in the container. And I left a TODO in the text.

@slp @stefano-garzarella @stsquad @jiangliu what projects should I mention here: https://github.com/andreeaflorescu/community/blob/improve_community_docs/README.md#why-rust-vmm?

[stefano-garzarella]

@andreeaflorescu maybe we can mention also QEMU that are re-using rust-vmm crates through vhost-user devices (e.g. virtiofsd, vhost-user-vsock and other devices available in the rust-vmm/vhost-device wroskpace)