docs: update README/docs for cryptography providers

[cpu]

As I understand things, since #444 there are no more first party aws-lc-rs or ring features on this crate, and instead you're expected to provide signature algorithms from the CryptoProvider struct's WebPkiSupportedAlgorithms. This branch updates the README and rustdoc to reflect that better.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.68%. Comparing base (a531696) to head (d2ea3c9).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #460   +/-   ##
=======================================
  Coverage   96.68%   96.68%           
=======================================
  Files          20       20           
  Lines        3896     3896           
=======================================
  Hits         3767     3767           
  Misses        129      129           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.