Add Disco to List of state management approaches

[manuel-plavsic]

Description of what this PR is changing or adding, and why:
Adds Disco to the list of state management approaches.
However, since Disco is actually a DI-only library (it complements state-management solutions, but it is not include any form of state management/reactivity), would it make more sense to include a new section under https://docs.flutter.dev/app-architecture/case-study/dependency-injection instead of adding Disco to the List of state management approaches?

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

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[sfshaza2]

/gcbrun

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[sfshaza2]

@manuel-plavsic, can you sign the CLA?

[sfshaza2]

Hi, @manuel-plavsic! Thanks for this PR, but we've updated the page on the site to refer folks to pub.dev, so I am closing this.