Add "Custom classes" section in develop documentation (#149)

* Add class documentation

* Update docs/en/modules/ROOT/nav.adoc

Co-authored-by: Andrés Pereira de Lucena <andreslucena@users.noreply.github.com>

---------

Co-authored-by: Andrés Pereira de Lucena <andreslucena@users.noreply.github.com>

docs/en/modules/ROOT/nav.adoc

@@ -156,6 +156,18 @@
 *** xref:develop:guide_architecture.adoc[5. Architecture]
 ** xref:develop:security.adoc[Security]
 ** Advanced
+*** Creating custom classes
+**** xref:develop:classes/cells.adoc[Cells]
+**** xref:develop:classes/commands.adoc[Commands]
+**** xref:develop:classes/controllers.adoc[Controllers]
+**** xref:develop:classes/events.adoc[Events]
+**** xref:develop:classes/forms.adoc[Forms]
+**** xref:develop:classes/jobs.adoc[Jobs]
+**** xref:develop:classes/mailers.adoc[Mailers]
+**** xref:develop:classes/models.adoc[Models]
+**** xref:develop:classes/permissions.adoc[Permissions]
+**** xref:develop:classes/presenters.adoc[Presenters]
+**** xref:develop:classes/queries.adoc[Queries]
 *** Concerns
 **** xref:develop:authorable.adoc[Authorable]
 **** xref:develop:embeddable.adoc[Embeddable]
@@ -189,7 +201,6 @@
 *** xref:develop:turbolinks.adoc[Turbolinks]
 *** Views
 **** xref:develop:content_blocks.adoc[Content Blocks]
-**** xref:develop:data-picker.adoc[Data Picker]
 **** xref:develop:newsletter_templates.adoc[Newsletter Templates]
 **** xref:develop:view_hooks.adoc[View Hooks]
 **** xref:develop:view_models_aka_cells.adoc[View Models (Cells)]