Rework jekyll-tree build to add redirects from site root.

As described in the commit message for 5596d3b, the new docs.bazel.build
site will have one directory per released version and a master directory
for the latest documentation at HEAD. This change reworks the
jekyll-tree script to construct a final Jekyll tree containing the
documentation at HEAD with documentation under /versions/master and
redirect pages at the site root for each documentation page.

TESTED=scripts/serve-docs.sh

RELNOTES: None
PiperOrigin-RevId: 159920553

site/docs/index.html

diff --git a/site/docs/index.html b/site/docs/index.html
new file mode 100644
index 0000000..b32fa5c
--- /dev/null
+++ b/site/docs/index.html
@@ -0,0 +1,4 @@
+---
+layout: redirect
+redirect: bazel-overview.html
+---