Export types & doc improvements (#2329)

* Export types used in app.py

* Export more linked types/errors/classes.

* Remove custom template.

* Address review comments.

We need to have explicit 'Returns:' sections in properties if we want to link to the return type while https://github.com/mkdocstrings/python/issues/65 is open.

* Improve docs.

docs/api/errors.md

@@ -0,0 +1 @@
+::: textual.errors

docs/api/filter.md

@@ -0,0 +1 @@
+::: textual.filter

docs/api/scrollbar.md

@@ -0,0 +1 @@
+::: textual.scrollbar

docs/api/types.md

@@ -0,0 +1 @@
+::: textual.types