Refactor the huge README.md into the more structured mkdocs.

I also cleaned up the installation section and added links to various
competing redo implementations.

The new README.md is basically just link to the docs on
readthedocs.org, and a link to the mailing list.

These docs need a *lot* more work, but this is enough of an improvement
that I'll commit it anyway for now.

mkdocs.yml

@@ -1,4 +1,4 @@
-site_name: redo build system
+site_name: the _redo_ build system
 theme: readthedocs
 docs_dir: Documentation
 site_dir: website
@@ -6,7 +6,16 @@ strict: true
 
 pages:
   - Introduction: index.md
-  - Man Pages:
+  - Getting Started: GettingStarted.md
+  - Cookbook.md
+  - FAQ:
+    - Basics: FAQBasics.md
+    - Semantics: FAQSemantics.md
+    - Interop with make: FAQInterop.md
+    - Parallel Builds: FAQParallel.md
+    - Implementation Details: FAQImpl.md
+  - Contributing.md
+  - Command Reference (man pages):
     - redo: redo.md
     - redo-ifchange: redo-ifchange.md
     - redo-ifcreate: redo-ifcreate.md