Add docs on manually-provided schemas

doc/endpoint/integrations.md

@@ -3,7 +3,7 @@
 ```eval_rst
 .. note::
 
-  Note that the codecs defined by the tapir integrations are used only when the specific types (e.g. enumerations0 are 
+  Note that the codecs defined by the tapir integrations are used only when the specific types (e.g. enumerations) are 
   used at the top level. Any nested usages (e.g. as part of a json body), need to be separately configured to work with 
   the used json library.
 ```

doc/endpoint/schemas.md

@@ -9,6 +9,9 @@ implicit scope during codec derivation, as well as when using [json](json.md) or
 Implicit schemas for basic types (`String`, `Int`, etc.), and their collections (`Option`, `List`, `Array` etc.) are
 defined out-of-the box. They don't contain any meta-data, such as descriptions or example values.
 
+There's also a number of [datatype integrations](integrations.md) available, which provide schemas for various 
+third-party libraries.
+
 For case classes and sealed hierarchies, `Schema[_]` values can be derived automatically using
 [Magnolia](https://github.com/softwaremill/magnolia), given that implicit schemas are available for all the case class's
 fields, or all of the implementations of the `enum`/`sealed trait`/`sealed class`.
@@ -18,6 +21,10 @@ Two policies of custom type derivation are available:
 * automatic derivation
 * semi automatic derivation
 
+Finally, schemas can be provided by hand, e.g. for Java classes, or other custom types. As a fallback, you can also
+always use `Schema.string[T]` or `Schema.binary[T]`, however this will provide only basic documentation, and won't
+perform any [validation](validation.md).
+
 ## Automatic derivation
 
 Schemas for case classes, sealed traits and their children can be recursively derived. Importing `sttp.tapir.generic.auto._`