docs: make docs build incremental

docs: make docs build incremental

This patch makes the `make docs` directive incremental
avoiding re-running the siphon when the source hasn't
changed, and leveraging sphinx internal cache.
It adds a `make rebuild-docs` directive for cases where
this caching logic might break, e.g. in CI.
The virtualenv doesn't also get recreated on each build,
which might be enough when writing docs, provided
automated process leverage its rebuild counterpart.

Type: improvement

Change-Id: Ie90de3adebeed017b249cad81c6c160719f71e8d
Signed-off-by: Nathan Skrzypczak <nathan.skrzypczak@gmail.com>
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>