bpo-30964: Mention ensurepip in package installation docs (GH-2786)

Adds a new 'Pip not installed' section that covers
running `ensurepip` manually, and also references
the relevant section of the Python Packaging User
Guide.
diff --git a/Misc/ACKS b/Misc/ACKS
index 3385269..e0640c1 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -809,6 +809,7 @@
 Lenny Kneler
 Pat Knight
 Jeff Knupp
+Nicholas Kobald
 Kubilay Kocak
 Greg Kochanski
 Manvisha Kodali