[formrecognizer] Initial work for adding get_children methods (Azure#21224)

* initial get_children work

* add support for cross page elements, update return types

* add tests

* add recordings

* update DocumentContentElement test

* move get children to DocumentKeyValueElement, typing fixes

* remove references to selection marks

* fix document key value element test

* remove long type annotation

* update changelog

* newline

* doc fixes and improvements

* rename test class

* refactor get children methods

* update wording for confidence

* fix document content element docs

* specify return type in get element list helper

* more doc fixes

* fix type return

* refactor get_children into separate methods

* update tests for new methods

* add docs

* update changelog

* remove extra param and check from internal _get_children

* transform words and lines to convenience layer versions

* add test

* pass named params

Author: catalinaperalta

sdk/formrecognizer/azure-ai-formrecognizer/CHANGELOG.md

@@ -3,6 +3,8 @@
 ## 3.2.0b2 (Unreleased)
 
 ### Features Added
+- Added `get_words` methods on `AnalyzedDocument`, `DocumentEntity`, `DocumentField`, `DocumentLine`, `DocumentKeyValueElement`, `DocumentTable`, `DocumentTableCell`.
+- Added `get_lines` methods on `AnalyzedDocument`, `DocumentEntity`, `DocumentField`, `DocumentKeyValueElement`, `DocumentTable`, `DocumentTableCell`.
 
 ### Breaking Changes
 - Renamed `DocumentElement` to `DocumentContentElement`.