Flutter docs improvements

[PeloWriter]

This PR moves the Flutter docs content into the common format and fixes some common content issues that surfaced during the fixing.

To do (@marandaneto will work with me on this):

• determine if we need a migration guide
• review the options list on Configurations > Basic Options and add to the content as needed
• add a code sample to configuration/decluttering on the Filtering page
• Clarify if the scopes content should display and add the missing code sample
• refine the Feature list

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[marandaneto]

we also Support Flutter Web, it's a bit limited compared to Android and iOS, of course.
Eg, events are not enriched with device context.

[PeloWriter]

I've added "Limited Support for Flutter Web" - LMK if there's a link to add or if that wording is quite what we want here @bruno-garcia

[marandaneto]

sounds good to me, so we don't create much expectations from Web devs as we didn't spend time on it

[marandaneto]

determine if we need a migration guide

ideally yes, migrating the Dart v3 to Dart v4 or Flutter v4, but we should not block this PR by that

[marandaneto]

add a code sample to configuration/decluttering on the Filtering page

we don't have any logging integration, for now, hence this is not needed

[marandaneto]

Clarify if the scopes content should display and add the missing code sample

it's good to go, we don't support Local scopes

[marandaneto]

configurations:

supported and missing:

auto-session-tracking
integrations
transport

I've noticed that these 3 are supported across the Mobile SDKs.
For the other platforms, we can work on it here #2985

[marandaneto]

@PeloWriter I left a few comments, looks great, thanks for doing that, wondering if we could expand these changes later on on the Dart SDK as well, pretty much the same structure, way nicer, thanks once more.

[marandaneto]

should we take a look at the wizard too? see if something is off after these changes or if something could be improved?

[marandaneto]

LGTM