doc: identify release version in generated docs

It's not obvious which kernel release version you're reading about in the
documentation.  Add the version info in the breadcrumb header (instead
of "Home / Docs / Subsystems /" show as
"Home / Docs / 1.8 / Subsystems /").

(Depends on docs-theme PR-9, but can be merged now with no ill-effect)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>