Versioned documentation

![ss](https://user-images.githubusercontent.com/347918/49976566-87573880-ff10-11e8-805f-33d8393a3c45.png)

Demo:
- https://bazel-docs-staging.netlify.com/versions/master/bazel-overview.html
- https://bazel-docs-staging.netlify.com/versions/0.19.1/bazel-overview.html

This PR introduces a pure-HTML version selection mechanism for pages
on docs.bazel.build.

It also introduces a non-destructive script that generates a
documentation tree from a git checkout of a release tag. The script is
currently manually run after every release, but it only needs to be run
once per release.

There are also additional manual modifications required to add the new
values into a couple of config files, which I will take on as an
important follow up.

Another useful follow up is to integrate versioned documentation generation
into Bazel's release pipeline.

For more details, read the design document:
https://docs.google.com/document/d/1MNe8IWz7td4_Yr3r43YL-hrFSmQvFutum-av-Sny56s/edit

Fixes #579
Also see #1788
Also see #3336
Also see #3537

RELNOTES[NEW]: https://docs.bazel.build now supports versioned
documentation. Use the selector at the top of the navigation bar to
switch between documentation for different Bazel releases.

Closes #6923.

Change-Id: I4641a2556985ec085a156430d8c4fbc40e3aae0a
PiperOrigin-RevId: 225563710
diff --git a/WORKSPACE b/WORKSPACE
index ef6fb68..907c1bb 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -278,3 +278,11 @@
       "jdk10-server-release-1804.tar.xz" : ["https://mirror.bazel.build/openjdk.linaro.org/releases/jdk10-server-release-1804.tar.xz"],
   },
 )
+
+load("//scripts/docs:doc_versions.bzl", "DOC_VERSIONS")
+
+[http_file(
+    name = "jekyll_tree_%s" % DOC_VERSION["version"].replace(".", "_"),
+    sha256 = DOC_VERSION["sha256"],
+    urls = ["https://mirror.bazel.build/bazel_versioned_docs/jekyll-tree-%s.tar" % DOC_VERSION["version"]],
+) for DOC_VERSION in DOC_VERSIONS]