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

Documentation/markdown.md

diff --git a/Documentation/markdown.md b/Documentation/markdown.md
index 3e0f392..fdd34a9 100644
--- a/Documentation/markdown.md
+++ b/Documentation/markdown.md
@@ -703,45 +703,5 @@
 ## Configuration
 
 The `gitiles.config` file supporting the site contains several
-configuration options that impact markdown rendering.
-
-### Disabling markdown
-
-Markdown can be completely disabled by setting render to false.
-
-```
-[markdown]
-  render = false
-```
-
-### Markdown size
-
-Markdown files are limited by default to 5 MiB of input text
-per file. This limit is configurable, but should not be raised
-beyond available memory.
-
-```
-[markdown]
-  inputLimit = 5M
-```
-
-### Image size
-
-Referenced [images are inlined](#Images) as base64 encoded URIs.
-The image limit places an upper bound on the byte size of input.
-
-```
-[markdown]
-  imageLimit = 256K
-```
-
-### Google Analytics
-
-[Google Analytics](https://www.google.com/analytics/) can be
-enabled on every rendered markdown page by adding the Property ID
-to the configuration file:
-
-```
-[google]
-  analyticsId = UA-XXXX-Y
-```
+configuration options that impact markdown rendering. Refer to the
+[configuration documentation](config.md#Markdown) for details.