Adaptation information for inputs and app bars (#8509)

This PR adds some information on how to adapt styling for input widgets
with .adaptive() constructors, as well as top app bars.

Note that I am not sure of the best way to style the tables or size the
images. Also, I have added some commented out sections that should be
added when stable release goes live.

Fixes: #8428

## Presubmit checklist
- [X] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [X] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [X] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/master/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.


@MitchellGoodwin could you take a quick peak at the code and make sure
it looks okay?

@InMatrix feel free to propose any edits!

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Co-authored-by: Shams Zakhour (ignore Sfshaza) <44418985+sfshaza2@users.noreply.github.com>

Author: 2 people