Move configuration documentation to a separate page

Add a new "Configuration" page. Move the existing documentation out
of the Markdown page.

Link to the new configuration documentation from the navbar, and
add a brief paragraph about it in README.

Change-Id: I76de97bcd459d55cc46fa8d6c6da9be1202c1297

navbar.md

diff --git a/navbar.md b/navbar.md
index dde7a44..608531c 100644
--- a/navbar.md
+++ b/navbar.md
@@ -1,3 +1,4 @@
 * [Home](/README.md)
 * [Markdown](/Documentation/markdown.md)
+* [Configuration](/Documentation/config.md)
 * [Developers](/Documentation/developer-guide.md)