[commands] Update docs to include proper walkthrough (#426)

* Add ability to parse Unions into ext.commands

* pipe was 3.10, not 3.8

* Add start of commands walkthrough

* Fix optional parsing when optional is the last argument

* Add Annotated support

* Run black

* Add more command walkthrough

* revamp errors with proper useful messages and details on objects

* Final touches to the walkthrough

* update changelog with changes

* add changelog entry

* run black

* update function names past cookie section

Author: IAmTomahawkx

docs/changelog.rst

@@ -31,6 +31,15 @@ Master
             - ``Optional[T]`` / ``T | None``
             - ``Annotated[T, converter]`` (accessible through the ``typing_extensions`` module on older python versions)
 
+- ext.commands
+    - Additions
+        - Added support for the following typing constructs in command signatures:
+            - ``Union[A, B]`` / ``A | B``
+            - ``Optional[T]`` / ``T | None``
+            - ``Annotated[T, converter]`` (accessible through the ``typing_extensions`` module on older python versions)
+
+- Docs
+    - Added walkthrough for ext.commands
 
 2.7.0
 ======