gh-97731: Specify the full path to the docs for `make docclean` (GH-98982)

Specify the full path to the docs for `make docclean`
This is to have `make clean` not error out on cross-builds.
(cherry picked from commit 22bab74c8f5fa6d53ddb7e40baa4334d09331c0f)

Co-authored-by: Brett Cannon <brett@python.org>