Update dio library doc

[mcmah309]

Not sure why these exist, they make the generated docs look squirrely https://pub.dev/documentation/dio/latest/dio/

[AlexV525]

Could you elaborate "squirrely"?

[mcmah309]

"Migration Guide" and "Plugins" point to the same location and are just the regular docs for the dio library. It does not make sense to have these as categories.

The side view currently looks like this -

Libraries
browser
io
  Migration Guide
  dio
  Plugins
  dio

While it should look like

Libraries
browser
dio
io

[AlexV525]

Maybe the problem is library dio should be library only.

[mcmah309]

I don't see how that would remove the unnecessary categories.

But we may have to remove

dio/dio/dartdoc_options.yaml

Line 4 in b16a611

"Migration Guide":

as well. I think the changes in this PR are enough though.

[AlexV525]

These are necessary topics IMO. Is there a proper way to keep them while removing the extra library entries?

[mcmah309]

I don't think the changes in this PR will remove the topics. Just the sub-categories under "Libraries". "Topics" should remain the same.

[AlexV525]

Unfortunately, the topics are gone after removing these categories. Maybe this should be tracked with a Dart doc issue?