mmdet3d Docs update for registry location

[pd-michaelstanley]

Registry docs are now under Advanced Tutorials.

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Motivation

Changed doc location. I was looking for the docs and had to go find them myself.

Modification

Updated the doc link for Registry in mmengine

BC-breaking (Optional)

No, no breaking changes

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 the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.

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All committers have signed the CLA.

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