Add documentation for first-child and last-child pseudo-classes

Author: Sergi Pons Freixes

docs/guide/CSS.md

@@ -343,10 +343,12 @@ Here are some other pseudo classes:
 - `:disabled` Matches widgets which are in a disabled state.
 - `:enabled` Matches widgets which are in an enabled state.
 - `:even` Matches a widget at an evenly numbered position within its siblings.
+- `:first-child` Matches a widget that is the first amongst its siblings.
 - `:first-of-type` Matches a widget that is the first of its type amongst its siblings.
 - `:focus-within` Matches widgets with a focused child widget.
 - `:focus` Matches widgets which have input focus.
 - `:inline` Matches widgets when the app is running in inline mode.
+- `:last-child` Matches a widget that is the last amongst its siblings.
 - `:last-of-type` Matches a widget that is the last of its type amongst its siblings.
 - `:light` Matches widgets in light themes (where `App.theme.dark == False`).
 - `:odd` Matches a widget at an oddly numbered position within its siblings.