docs: add mobx_utils page

[amondnet]

Describe the changes proposed in this Pull Request.

If the PR fixes a specific issue, reference the issue with Fixes #.

---

Pull Request Checklist

• [x] If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• [x] Increment the major/minor/patch/patch-count, depending on the complexity of change
• [x] Add the necessary unit tests to ensure the coverage does not drop
• [x] Update the Changelog to include all changes made in this PR, organized by version
• [x] Run the melo run set_version command from the root directory
• [x] Include the necessary reviewers for the PR
• [x] Update the docs if there are any API changes or additions to functionality

[codecov[bot]]

Codecov Report

Merging #893 (bc9a4f9) into main (d7dbbf5) will not change coverage. The diff coverage is n/a.

Additional details and impacted files

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[fzyzcjy]

P.S. The title makes me remind of https://github.com/mobxjs/mobx.dart/issues/750 where it is suggested to be put into sth like mobx_extension

[amondnet]

@fzyzcjy I got the name from https://github.com/mobxjs/mobx-utils.

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