## Description
This PR adds more documentation for `TextEditingController(String text)` constructor and it adds one example.
https://github.com/flutter/flutter/pull/96245 was a first improvement to the documentation.
https://github.com/flutter/flutter/issues/79495 tried to hide the cursor when an invalid selection is set but it was reverted.
https://github.com/flutter/flutter/pull/123777 mitigated the issue of having a default invalid selection: it takes care of setting a proper selection when a text field is focused and its controller selection is not initialized.
I will try changing the initial selection in another PR, but It will probably break several existing tests.
## Related Issue
Fixes https://github.com/flutter/flutter/issues/95978
## Tests
Adds 1 test for the new example.
* Add support for image insertion on Android
* Fix checks
* Use proper Dart syntax on snippet
* Specify type annotation on list
* Fix nits, add some asserts, and improve example code
* Add missing import
* Fix nullsafety error
* Fix nullsafety error
* Remove reference to contentCommitMimeTypes in docs
* Fix nits
* Fix warnings and import
* Add test for content commit in editable_text_test.dart
* Check that URIs are equal in test
* Fix nits and rename functions / classes to be more self-explanatory
* Fix failing debugFillProperties tests
* Add empty implementation to `insertContent` in TextInputClient
* Tweak documentation slightly
* Improve docs for contentInsertionMimeTypes and fix assert
* Rework contentInsertionMimeType asserts
* Add test for onContentInserted example
* Switch implementation to a configuration class for more granularity in setting mime types
* Fix nits
* Improve docs and fix doc tests
* Fix more nits (LongCatIsLooong)
* Fix failing tests
* Make parameters (guaranteed by platform to be non-nullable) non-nullable
* Fix analysis issues
