mkdocs: enforce sufficiently new version, and use mkdocs-exclude.

We want to use the mkdocs-exclude plugin, which lets us exclude
particular files from the output directory.  But plugins aren't
available in the debian-stable version of mkdocs, so ensure that we're
running a sufficiently new version.  If we aren't, gracefully just skip
building the documentation.

mkdocs.yml

@@ -9,8 +9,20 @@ extra_javascript:
   - fetchcode.js
 extra_css:
   - extra_style.css
+plugins:
+  - exclude:
+      glob:
+        - "t/*"
+        - "*.tmp"
+        - "*.gz"
+        - "*.tar"
+        - "*.pdf"
+        - "*.ps"
+        - "*.1"
+      regex:
+        - ".*~"
 
-pages:
+nav:
   - Introduction: index.md
   - Getting Started: GettingStarted.md
   - Contributing.md