Hint users when having references in output type, hint when wrong rust_input, add flutter_package example, and improve docs

[fzyzcjy]

Changes

Close #2083
Close #2084
Close #1969

Checklist

• An issue to be fixed by this PR is listed above.
• New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• If this PR adds/changes features, documentations (in the ./website folder) are updated.
• CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.11%. Comparing base (39c1f80) to head (95e11b2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2100      +/-   ##
==========================================
+ Coverage   98.41%   99.11%   +0.70%     
==========================================
  Files         485      485              
  Lines       19597    19635      +38     
==========================================
+ Hits        19286    19461     +175     
+ Misses        311      174     -137     

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