documentation rework

[bmoffatt]

Description of changes:

Moved stuff around to look a bit nicer in https://pkg.go.dev/ and go tool doc.

• Examples in event/README_*.md moved to example_*_test.go, so that they show up adjacent to their types.
• Inline block examples in lambda/handler.go moved to lambda/example_test.go
• cfn/README.md documentation and example moved to cfn/doc.go and cfn/example_test.go
• Added an example for a io.Reader handler, and link to response streaming documentation.
• Added events/doc.go, lambda/doc.go and lambdaurl/doc.go and additional doc comments to lambdacontext/context.go
• Removed any empty README.md that just linked to pkg.go.dev

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.59%. Comparing base (de51f68) to head (eca4dd5).
⚠️ Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
+ Coverage   73.30%   73.59%   +0.29%     
==========================================
  Files          26       35       +9     
  Lines        1487     1337     -150     
==========================================
- Hits         1090      984     -106     
+ Misses        324      277      -47     
- Partials       73       76       +3     

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