docs: Doc edits and some reorg

[crandmck]

• Move info on running tests into Project Contributions (for SDK devs vs. consumers)
• General copy edit of README.

Questions:

• We should provide some link for further info on StrongBox. Is this the best ref? https://developer.android.com/privacy-and-security/keystore#StrongBoxKeyMint

Types of changes

Docs only.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.78%. Comparing base (d09bd35) to head (d9f4ae5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #63   +/-   ##
=========================================
  Coverage     65.78%   65.78%           
  Complexity        9        9           
=========================================
  Files            56       56           
  Lines          1920     1920           
  Branches        218      218           
=========================================
  Hits           1263     1263           
  Misses          510      510           
  Partials        147      147           

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[redaranj]

Looks good - thanks!