doc: add sphinx-tabs extension

The sphinx-tabs extension provides a tabbed interface within documents
that will let us dynamically display information based on a user
selection, for example, for instruction variations based on the user's
development OS (Linux, macOS, Windows).

I'm adding this early so it can get integrated into the CI doc build
before subsequent PRs using this extension are submitted.

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