Rewrite python docs & onboarding to be more opinionated

* The docs now explicitly tell people how to write UI scripts, then UI
  extensions, with as quick a start as possible, and go into detail in later
  documents. Use of the python module directly is also not featured prominently
  as this is a rare use-case for casual scripting needs.

docs/python_api/first_steps.py

@@ -0,0 +1,27 @@
+if not pyrenderdoc.IsCaptureLoaded():
+    filename = pyrenderdoc.Extensions().OpenFileName("Choose a capture", "", "*.rdc")
+
+    pyrenderdoc.LoadCapture(filename, renderdoc.ReplayOptions(), filename, False, True)
+
+eid = pyrenderdoc.CurEvent()
+
+name = pyrenderdoc.GetEventBrowser().GetEventName(eid)
+
+print(f"Currently we are at EID {eid} named: '{name}'")
+if eid > 1:
+    prevname = pyrenderdoc.GetEventBrowser().GetEventName(eid - 1)
+    print(f"    > the previous event {eid-1} is named: '{prevname}'")
+
+pipe = pyrenderdoc.CurPipelineState()
+
+outputs = pipe.GetOutputTargets()
+
+for idx, out in enumerate(outputs):
+    if out.resource != renderdoc.ResourceId.Null():
+        name = pyrenderdoc.GetResourceName(out.resource)
+        print(f"Output {idx} is: {name}")
+
+depth = pipe.GetDepthTarget()
+
+name = pyrenderdoc.GetResourceName(depth.resource)
+print(f"Depth is: {name}")