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

README.md

diff --git a/README.md b/README.md
index 3bbfb98..4bb030d 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,12 @@
 documentation. Refer to the [Markdown documentation](/Documentation/markdown.md)
 for details.
 
+## Configuration
+
+Gitiles is configurable in a git-style configuration file named
+`gitiles.config`. Refer to the [configuration documentation](/Documentation/config.md)
+for details.
+
 ## Contributing to Gitiles
 
 Please refer to the [Developer Guide](/Documentation/developer-guide.md).