Rewrite the README document

[Turbo87]

---

Previously, about 50% of our README document was dedicated to "Categories", while missing quite a few more relevant parts. This PR updates our README with:

• a nice logo at the top
• a couple of quicklinks to our policies, the status page, and other important places
• an short overview of what crates.io is
• a sentence about our primary infrastructure sponsors
• an explanation about our split between GitHub Issues (for bugs) and Discussions (for feature proposals/requests)
• a "Code of Conduct" section

A rendered preview is available at https://github.com/Turbo87/crates.io/blob/readme/README.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.56%. Comparing base (8521cee) to head (022c2e2).

❗ Current head 022c2e2 differs from pull request most recent head 907c146. Consider uploading reports for the commit 907c146 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8214   +/-   ##
=======================================
  Coverage   87.56%   87.56%           
=======================================
  Files         271      271           
  Lines       26983    26983           
=======================================
  Hits        23628    23628           
  Misses       3355     3355           

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[LawnGnome]

I like this!

A couple of suggestions, but you can take or leave them as you wish.