Updating documentation on UIScene migration

[vashworth]

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

Flutter is about to add support for UIScene and this documentation will go along with it.

Issues fixed by this PR (if any):

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

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• 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 pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[vashworth]

Not all of the UIScene changes have landed yet, so we're not quite ready to land this yet.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 557397e):

https://flutter-docs-prod--pr12490-uiscene-documentation-g1dip65r.web.app

[sfshaza2]

I know this is still a draft, but I did some text editing.

[sfshaza2]

lgtm