Link to topic on pub.dev for state management packages

[parlough]

Rather than trying to maintain a list of state management packages and their relevant resources, or just accepting whichever is contributed as we've historically done, let's just link to a pub.dev topic instead.

This PR updates the state-management approaches page to:

• Group the built-in, low-level solutions under a new header.
• Remove each of the listed third-party packages.
• Add a new section about community-provided packages.
  • Briefly introduces why you might want to use one.
  • Highlights the options are provided by the community.
  • Clarifies there is no right choice, but it depends on each app and team's needs.
  • Links to the #state-management topic on pub.dev.
  • Links again to the topic with a prominent link card.
  • Offers a suggestion to add to the list and links to the relevant documentation on dart.dev.

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Staged: https://flutter-docs-prod--pr12368-feat-12252-uy54axns.web.app/data-and-backend/state-mgmt/options#community-provided-packages

Resolves #12252

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit d86e7db):

https://flutter-docs-prod--pr12368-feat-12252-uy54axns.web.app

[sfshaza2]

I love this, @parlough! We should also adopt this on this [iOS page(https://github.com//pull/12263). (Note that I'm not asking you to do it, but we've been discussing this in an internal chat. I'm all for more robust solutions that require less time to maintain...)