Move long README sections to the book, to make it easier for new users to get started with Zebra

[mpguerra]

Is your feature request related to a problem? Please describe.

From our user studies we discovered that users found the README a bit long and overwhelming and that, after skimming the README, they still asked us about information that was present within.

Describe the solution you'd like

Some ideas/suggestions:

• Remove information about First Alpha release in favour of some banner or something else that clearly shows that Zebra is currently at Alpha stage.
• Move "System Requirements" and "Network Requirements" to "Getting Started" section below:
  "Building zebrad requires Rust, libclang, and a C++ compiler." and above "Detailed Build and Run Instructions"
• Expand on Build and run instructions to include explicit and specific about required package managers for various operating systems and other environmental setup, binaries, etc...
• Add hints to the "Detailed Build and Run Instructions" section about making sure cargo is in path
• Link out to Zebra book and provide high level headlines on sections such as "Architecture"

Describe alternatives you've considered

Leave the README as is.

Additional context

This was discovered during our first alpha user studies (#1501).

[teor2345]

This was on the retrospective agenda today, so I'm going to leave it open.