Update "Web embedding" documentation.

[ditman]

This PR:

• Adds an "Embedding Flutter on the Web" documentation page describing the different ways that Flutter content can be added to a host website: full-screen, embedded mode/multiView, custom element (single-view embedded mode).
• Adds a "Embedding web content into a Flutter web app" documentation page describing how to add Web content into a Flutter web app as a Widget.

(Both of the pages are added under the "Platform integration > Web" section.)

The PR also:

• De-duplicates redundant embedding content from different pages, by linking directly to the new documents (Web FAQ, Web deployment)
• Adds an "Add to a web app" link under the "Add to an existing app" menu section, pointing to the Embedding on the web, so people looking at Add To App finds the Web docs more easily.

Issues

Fixes: #10773

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.

[ditman]

/cc @johnpryan @kevmoo @yjbanov FYI!

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit bb0e06f):

https://flutter-docs-prod--pr10977-web-embedding-docs-o3h28ltv.web.app

[kevmoo]

This seems SUPER complex. It feels like there should be a widget we ship here to do most of this heavy lifting.

[ditman]

MultiViewApp is (one of the) widgets you're describing; this is how it's used:

• https://github.com/goderbauer/mvp/blob/main/lib/widgets_counter.dart#L9-L11

(The viewBuilder function can (must?) have additional complexity to decide what widgets go into each viewId)

[sfshaza2]

Thanks SO MUCH, @ditman, for this PR! I've done some changes, mostly to make the language less passive. Once you process my feedback, lgtm.

[sfshaza2]

We're getting there!

[sfshaza2]

Land when ready, @ditman!

[ditman]

Talked to @kevmoo, we're clicking the button!

[ditman]

This is now published, thanks!

• https://docs.flutter.dev/platform-integration/web/embedding-flutter-web
• https://docs.flutter.dev/platform-integration/web/web-content-in-flutter