[cheshire] Enhance documentation for Ara integration under Cheshire SoC

[jorgeloopzz]

Recently, trying to get Ara working under Cheshire SoC, I've stumbled upon some problems that I solve here.

1. Python's dependencies. Both, Ara and Cheshire, use their own Python packages, then I see it useful to have them all gathered in one file as I've done in requirements.txt (69554ce36341311a61554eee3364314b303c1c90)
2. Documentation. Because of this, I've had to be navigating between their documentations, which can be quite annoying and confusing some times; then, I've listed all the steps involved in compiling both platforms (a07bab0a6553c6473c58d49e7e6c9edaece960f4). Furthermore, I've specified the branch that includes the Ara inside Cheshire's repository (ffc8f7c5112b8abb8ec857c5c1cf1d1a5a0bd861); other branches would show the next error right after executing bender path ara

Error: Dependency `ara` does not exist. Did you forget to add it to the manifest?

Changelog

Changed

Cheshire documentation focusing on dependencies and workflow

Checklist

• Automated tests pass
• [ x] Changelog updated
• Code style guideline is observed