DOC-3202: Add migration guides for users migrating from TinyMCE 4,5 to v7.

Author: kemister85

modules/ROOT/nav.adoc

@@ -209,6 +209,8 @@
 **** xref:bundling-localization.adoc[UI localizations]
 **** xref:bundling-themes.adoc[Themes]
 ** xref:upgrading.adoc[Upgrading {productname}]
+** xref:migration-from-4x.adoc[Migrating from {productname} 4]
+** xref:migration-from-5x.adoc[Migrating from {productname} 5]
 ** xref:migration-from-6x.adoc[Migrating from {productname} 6]
 ** xref:migration-from-froala.adoc[Migrating from Froala]
 ** xref:generate-rsa-key-pairs.adoc[Generate public key pairs]

modules/ROOT/pages/migration-from-4x.adoc

@@ -0,0 +1,7 @@
+= Migrating from {productname} 4 to {productname} 7
+:navtitle: Migrating from TinyMCE 4
+:description: Guidance for migrating from TinyMCE 4 to TinyMCE 7
+:keywords: migration, considerations, premigration, pre-migration
+:release-version: 7.0
+
+// Todo

modules/ROOT/pages/migration-from-5x.adoc

@@ -0,0 +1,7 @@
+= Migrating from {productname} 5 to {productname} 7
+:navtitle: Migrating from TinyMCE 5
+:description: Guidance for migrating from TinyMCE 5 to TinyMCE 7
+:keywords: migration, considerations, premigration, pre-migration
+:release-version: 7.0
+
+// Todo