Add Phriction to the main nav menu

Summary:
A few minor things:

  - Add Phriction to the homepage.
  - Link to remarkup reference on the edit screen.
  - Add a help tab with a super basic help document.

Test Plan: Cursory inspection.
Reviewed By: codeblock
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, codeblock
Differential Revision: 655

src/docs/userguide/phriction.diviner

@@ -0,0 +1,12 @@
+@title Phriction User Guide
+@group userguide
+
+Construct a detailed written history of your civilization.
+
+= Overview =
+
+Phriction is a simple wiki. You can edit pages, and the text you write will stay
+there. Other people can see it later.
+
+NOTE: Phriction is extremely new and very basic for now. Give us feedback on
+what you'd like to see improve!

src/docs/userguide/remarkup.diviner

@@ -141,4 +141,15 @@ In Differential and Maniphest, you can mention another user by writing:
   @username
 
 When you submit your comment, this will add them as a CC on the revision or task
-if they aren't already CC'd.
+if they aren't already CC'd.
+
+= Phriction Documents =
+
+You can link to Phriction documents with a name or path:
+
+  Make sure you sign and date your [legal/Letter of Marque and Reprisal]!
+
+With a pipe (##|##), you can retitle the link. Use this to mislead your
+opponents:
+
+  Check out these [legal/boring_documents/ | exciting legal documents]!