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/config.md

diff --git a/Documentation/config.md b/Documentation/config.md
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index 0000000..2779dd7
--- /dev/null
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+# Configuration
+
+The `gitiles.config` file supporting the site contains several configuration
+options.
+
+[TOC]
+
+## Markdown
+
+### 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
+```