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

site/docs/query.md

diff --git a/site/docs/query.md b/site/docs/query.md
new file mode 100644
index 0000000..7a42ae7
--- /dev/null
+++ b/site/docs/query.md
@@ -0,0 +1,4 @@
+---
+layout: redirect
+redirect: docs/query.html
+---