doc: add graphviz sphinx extension

This enables textual graphing to enhance the diagrams of the
documentation
doc: remove PGHOST references
doc: adding a summary of the architecture
doc: adding a very rough architecture diagram
doc: adding a blurb on a standard install

Following the tutorial is usually very error-prone if the user is new to
Munin, so he should not struggle with different OS configurations in
addtition.
doc: adding the tuto for a node
doc: fix table formating