Docs and more tests

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Codecov Report

Attention: Patch coverage is 75.55556% with 22 lines in your changes missing coverage. Please review.

Project coverage is 91.48%. Comparing base (c25505b) to head (ba399bf).

Files with missing lines	Patch %	Lines
packages/dartcv/lib/src/dnn/dnn.dart	27.77%	13 Missing :warning:
packages/dartcv/lib/src/core/rect.dart	80.43%	9 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #270 +/- ## ========================================== + Coverage 90.55% 91.48% +0.93% ========================================== Files 49 49 Lines 9049 9125 +76 ========================================== + Hits 8194 8348 +154 + Misses 855 777 -78 ``` | [Flag](https://app.codecov.io/gh/rainyl/opencv_dart/pull/270/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/rainyl/opencv_dart/pull/270/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu) | `91.48% <75.55%> (+0.93%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=rainy+liu#carryforward-flags-in-the-pull-request-comment) to find out more.

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