Replace doc pages with redirects to versioned doc pages.

* Add a new `redirect` Jekyll layout.
* Replace all pages under docs/ with redirects to corresponding page under
  versions/master/.
* Prepend links on Documentation sidebar, including generated navs for the
  Skylark Library and Build Encyclopedia, with prefix for versioned directory.
* Add code to both the internal jekyll-config.sh and external jekyll-tree.sh
  to add redirect pages for the Skylark Library and Build Encyclopedia.
* Bring the branched User Manual doc up to date with latest changes.

--
MOS_MIGRATED_REVID=131568800
diff --git a/site/BUILD b/site/BUILD
index 89cfc89..52f9da1 100644
--- a/site/BUILD
+++ b/site/BUILD
@@ -2,7 +2,7 @@
 
 exports_files(
     [
-        "docs/bazel-user-manual.html",
+        "versions/master/docs/bazel-user-manual.html",
         "command-line-reference-prefix.html",
         "command-line-reference-suffix.html",
     ],