README: Make it more beginner-friendly

[n0toose]

In the beginning, I found myself quite lost as I was not totally sure how to use Uhyve. This change introduces some additional background information and pointers for people looking to hack on the source code.

• Make "code blocks" more consistent
• Make some wording more consistent
• Additional Markdown magic
• Cut the length of some sentences
• Add cargo run method for developers
• Introduce GitHub-styled admonition boxes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 68.17%. Comparing base (e6ffffa) to head (2349887). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #714 +/- ## ======================================= Coverage 68.17% 68.17% ======================================= Files 20 20 Lines 2319 2319 ======================================= Hits 1581 1581 Misses 738 738 ```

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[n0toose]

I addressed the review comments and made some additional changes to make them "fit" well, such as adding clearer headers and separating the content better - this introduces some new changes without changing the substance.

Perhaps it would be better if a CONTRIBUTING.md file were to be written in the future instead of cramming everything in the README file, but it should be adequate enough for now.

[jounathaen]

Oh, and I'd stash these changes into a single commit...

[n0toose]

Wait, one more rebase!