Add support for custom columns on catalog categories (#4708)

docs/extensions/guides/catalog.md

@@ -1,5 +1,27 @@
 # Catalog (WIP)
 
-## CatalogCategoryRegistry
+This guide is a brief overview about how the catalog works within Lens.
+The catalog should be thought of as the single source of truth about data within Lens.
 
-## CatalogEntityRegistry
+The data flow is unidirectional, it only flows from the main side to the renderer side.
+All data is public within the catalog.
+
+## Categories
+
+A category is the declaration to the catalog of a specific kind of entity.
+It declares the currently supported versions of that kind of entity but providing the constructors for the entity classes.
+
+To declare a new category class you must create a new class that extends [Common.Catalog.CatalogCategory](../api/classes/Common.Catalog.CatalogCategory.md) and implement all of the abstract fields.
+
+The categories provided by Lens itself have the following names:
+
+- `KubernetesClusters`
+- `WebLinks`
+- `General`
+
+To register a category, call the `Main.Catalog.catalogCategories.add()` and `Renderer.Catalog.catalogCategories.add()` with instances of your class.
+
+## Entities
+
+An entity is the data within the catalog.
+All entities are typed and the class instances will be recreated on the renderer side by the catalog and the category registrations.