Rename noise_psk to key in ESPHome

[puddly]

To be consistent with Home Assistant discovery naming, rename noise_psk to key. Both are still accepted. I've also added a small how-to document to the docs with self-contained scripts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.85%. Comparing base (95b5860) to head (14f00d9).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #63      +/-   ##
==========================================
- Coverage   89.86%   89.85%   -0.01%     
==========================================
  Files          19       19              
  Lines        3010     3017       +7     
==========================================
+ Hits         2705     2711       +6     
- Misses        305      306       +1     

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