test-documentation: Invoke sphinx-build via make instead of directly.
test-documentation: Invoke sphinx-build via make instead of directly.

This lets us pick up the -W option from docs/Makefile.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulipchat.com>

punchagan/zulip,shubhamdhama/zulip,kou/zulip,eeshangarg/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,showell/zulip,rht/zulip,brainwane/zulip,timabbott/zulip,hackerkid/zulip,synicalsyntax/zulip,punchagan/zulip,shubhamdhama/zulip,hackerkid/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,brainwane/zulip,showell/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,brainwane/zulip,andersk/zulip,showell/zulip,shubhamdhama/zulip,shubhamdhama/zulip,showell/zulip,andersk/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,andersk/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,synicalsyntax/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,punchagan/zulip,rht/zulip,rht/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,synicalsyntax/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,showell/zulip,showell/zulip,hackerkid/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,eeshangarg/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,shubhamdhama/zulip,punchagan/zulip,brainwane/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,synicalsyntax/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,rht/zulip,andersk/zulip