[CodeCamp]Add doc of yolov6_description.md

[wzr-skn]

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Motivation

Please describe the motivation for this PR and the goal you want to achieve through this PR.

Modification

Add the Network Structure section doc of yolov6_description.md

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a potential influence on downstream projects, this PR should be tested with downstream projects, like MMDetection or MMClassification.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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[hhaAndroid]

@wzr-skn please sign the CLA

[PeterH0323]

Hi @wzr-skn please sign the CLA

[OpenMMLab-Assistant001]

Hi @wzr-skn ！First of all, we want to express our gratitude for your significant PR in the MMYOLO project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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