Add generated docs to Jekyll tree build target. Add script for bringing up
local instance of bazel.io site.

* Add a new genrule rule that runs the new jekyll-tree.sh to do the following:
  * Combine the generated docs for the Build Encyclopedia and Skylark Library
    with the static site docs
  * Combine the README.md files for the Docker and Packaging rules with the
    Jekyll tree
  * Process the generated docs, replaces instances of "blaze" with "bazel",
    etc.
* Add scripts/serve-docs.sh script that can be used to bring up a local
  instance of the bazel.io website.

As of this patch, it is possible to construct the full Bazel.io site tree from
the master branch.

--
MOS_MIGRATED_REVID=121813688
diff --git a/site/BUILD b/site/BUILD
index 2cc9736..d1107bc 100644
--- a/site/BUILD
+++ b/site/BUILD
@@ -64,7 +64,7 @@
 )
 
 pkg_tar(
-    name = "jekyll-tree",
+    name = "jekyll-base",
     deps = [
         ":bootstrap-css",
         ":bootstrap-images",
@@ -74,3 +74,31 @@
         ":jekyll-files",
     ],
 )
+
+pkg_tar(
+    name = "skylark-rule-docs",
+    files = [
+        "//tools/build_defs/docker:README.md",
+        "//tools/build_defs/pkg:README.md",
+    ],
+    strip_prefix = "/tools/build_defs",
+)
+
+genrule(
+    name = "jekyll-tree",
+    srcs = [
+        ":jekyll-base",
+        ":skylark-rule-docs",
+        "//src/main/java/com/google/devtools/build/lib:gen_buildencyclopedia",
+        "//src/main/java/com/google/devtools/build/lib:gen_skylarklibrary",
+    ],
+    outs = ["jekyll-tree.tar"],
+    cmd = ("$(location jekyll-tree.sh) $@ " +
+           "$(location :jekyll-base) " +
+           "$(location :skylark-rule-docs) " +
+           "$(location //src/main/java/com/google/devtools/build/lib:gen_buildencyclopedia) " +
+           "$(location //src/main/java/com/google/devtools/build/lib:gen_skylarklibrary)"),
+    tools = [
+        "jekyll-tree.sh",
+    ],
+)