fix(docs): refactor poetry dependency group

Rename 'doc' group to 'docs' for consistency, this is to follow the
widely accepted convention of using plural nouns groups that contain
multiple items. This change signifies that the 'docs' group includes a
collection of documentation-related dependencies.

Also, ensure that the dependencies are actually conditionally installed
by setting the group as optional. This was missing in the original
change.

Change-Id: I07caccfb1b57bc2dc1e7596899dfb926e8a5f71a
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
2 files changed