Request: Update the User Guide to mention more *nix systems

[rami3l]

Hello again! This is a minor feedback based on our past interactions such as #476 :)

Problem:

Build-related instructions remain unclear for *nix platforms outside of Linux/macOS/iOS, such as FreeBSD and Illumos.

Solution:

Reorganize https://aws.github.io/aws-lc-rs/requirements/index.html so that supported *nix platforms are appropriately covered.

Admittedly, including an exhaustive introduction of build-time dependencies in different scenarios could sound impractical.
OTOH, some hints/rationale may be included to make it easier for newcomers to deduce what dependencies are required for their build scenario (cross or not).