chore(release): prepare v0.6.5 release

[leehack]

Summary

• bump package version to 0.6.5 in pubspec.yaml
• move current release notes into the 0.6.5 section and reset docs Unreleased notes
• update installation snippets in README and website docs to reference ^0.6.5

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm -j 1 --exclude-tags local-only
• ./tool/docs/build_site.sh
• ./tool/docs/validate_links.sh

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.81%. Comparing base (33338c6) to head (7a8f666).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #77   +/-   ##
=======================================
  Coverage   76.81%   76.81%           
=======================================
  Files          66       66           
  Lines        8579     8579           
=======================================
  Hits         6590     6590           
  Misses       1989     1989           

Flag	Coverage Δ
unittests	76.81% <ø> (ø)

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