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 #.

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Pull Request Checklist

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

[codecov]

Codecov Report

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #893   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          57       57           
  Lines        1998     1998           
=======================================
  Hits         1978     1978           
  Misses         20       20           

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	98.55% <ø> (ø)
mobx_codegen	100.00% <ø> (ø)

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[fzyzcjy]

P.S. The title makes me remind of #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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