build(poetry): wrap docs build in poetry

Simplify building documentation by wrapping the Sphinx command inside
a Poetry environment.  Previously, the developer had to manually
activate Poetry and install the docs group.  This change makes the
process consistent with other build targets and reduces user error.
Falls back gracefully if Poetry is not installed.

Change-Id: I1dfea58314253773bec5e2c70d27e2216e3143d9
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
2 files changed