Remove 'Sections & Trees' and 'Searchable Trees' docs (#7634)

Deleted the 'section-trees.md' and 'searchable-trees.md' documentation files from versions 15, 16, and 17 of Umbraco CMS, and removed their references from the SUMMARY.md files.

Author: madsrasmussen

15/umbraco-cms/SUMMARY.md

@@ -210,8 +210,6 @@
   * [Validation](customizing/foundation/validation/README.md)
     * [Form Control Mixin](customizing/foundation/validation/form-control-mixin.md)
     * [Understand the Validation system](customizing/foundation/validation/integrate-validation.md)
-* [Sections & Trees](customizing/section-trees.md)
-* [Searchable Trees (ISearchableTree)](customizing/searchable-trees.md)
 * [Property Editors](customizing/property-editors/README.md)
   * [Property Editor Validation](customizing/property-editors/property-editor-validation.md)
   * [Property Editors Composition](customizing/property-editors/composition/README.md)

15/umbraco-cms/customizing/searchable-trees.md

@@ -215,11 +215,9 @@
   * [Integrate Validation](customizing/foundation/integrate-validation.md)
 * [Contexts](customizing/foundation/contexts/README.md)
   * [Property Dataset Context](customizing/foundation/contexts/property-dataset-context.md)
-* [Sections & Trees](customizing/section-trees.md)
 * [Property Level UI Permissions](customizing/property-level-ui-permissions.md)
 * [Icons](customizing/foundation/icons.md)
 * [BackOffice Signs](customizing/back-office-signs.md)
-* [Searchable Trees (ISearchableTree)](customizing/searchable-trees.md)
 * [Property Editors](customizing/property-editors/README.md)
   * [Property Editor Validation](customizing/property-editors/property-editor-validation.md)
   * [Property Editors Composition](customizing/property-editors/composition/README.md)