Add Flutter Forum as a support option (#11350)

_Description of what this PR is changing or adding, and why:_
Add https://forum.itsallwidgets.com/ as a support option

_Issues fixed by this PR (if any):_
n/a

_PRs or commits this PR depends on (if any):_
none

## Presubmit checklist

- [x] This PR is marked as draft with an explanation if not meant to
land until a future stable release.
- [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/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

src/content/resources/support.md

@@ -22,6 +22,7 @@ Note that filing bugs is _not_ an efficient way to get tech support.
 ## I want to ask questions about how to use Flutter
 
 Your best bet is to consult
+[Flutter Forum](https://forum.itsallwidgets.com/),
 [Stack Overflow](https://stackoverflow.com/questions/tagged/flutter),
 or [Discord](https://discord.com/invite/rflutterdev).
 While the community is active in these channels,