FWE pt 1

[ericwindmill]

Description of what this PR is changing or adding, and why:

This silently adds the "UI 101" pages of the upcoming FWE tutorial. This PR does not include any changes the side nav.

Part of #12063

Pages

• introduction
• create an app
• widgets
• layout
• devtools
• user interaction
• stateful widget
• implicit animations

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 346fd6d):

https://flutter-docs-prod--pr12035-flutter-tutorial-p25mm14a.web.app

[ericwindmill]

@Sfshaza @antfitch @parlough

For now, my goal is to get reviews on the text, and thus I haven't included images or diagrams, nor have I added the full code to CI. I think all of those things will need to be considered holistically once we know what the page is going to look like.