Lots more Document data definition

Schedule updates:

* Defined Schedule types
* Updated schedule loader to validate with pydantic
* Added ability to specify predecessor type and lead/lag

Other structural/outline stuff as well

Oh and added a unit test.

documents/_static/css/custom.css

@@ -12,3 +12,9 @@ html[data-theme="dark"] {
     --pst-color-primary: rgb(115, 199, 164);
     --pst-color-secondary: rgb(21, 197, 124);
 }
+
+/* Additional customizations for nrsk */
+dl.py.attribute {
+    /* Reduce the space between autodoc attrs */
+    margin-bottom: 0.5em !important; 
+}