Add the Welcome widget to the documentation

While it's arguably not the most useful widget, it's in the code, exported,
and can be seen by people; I feel it should be in the documentation. I've
ensured that it also links to Placeholder given it sort of is a Placeholder
and so people might arrive at it first and it makes sense to redirect them
to something more comprehensive.

docs/api/welcome.md

@@ -0,0 +1 @@
+::: textual.widgets.Welcome

mkdocs-nav.yml

@@ -182,6 +182,7 @@ nav:
     - "api/tree_node.md"
     - "api/tree.md"
     - "api/walk.md"
+    - "api/welcome.md"
     - "api/widget.md"
   - "Blog":
     - blog/index.md