Ensure dependencies are installed when building docs

[radarhere]

Moves the sphinx requirements from

Pillow/.github/workflows/test.yml

Line 102 in af8260e

python3 -m pip install furo sphinx-copybutton sphinx-issues sphinx-removed-in sphinxext-opengraph

to

Pillow/docs/Makefile

Lines 45 to 47 in af8260e

	install-sphinx:
	$(PYTHON) -c "import sphinx" > /dev/null 2>&1 || $(PYTHON) -m pip install sphinx
	$(PYTHON) -c "import furo" > /dev/null 2>&1 || $(PYTHON) -m pip install furo

so that make doc can ensure that all of the dependencies are installed.

Also ensures that Pillow and olefile are installed, two more requirements for building the docs.

[hugovk]

We're building Pillow anyway, shall we just use the docs extra?

Suggested change

	python3 -c "import PIL" > /dev/null 2>&1 || python3 -m pip install .
	python3 -c "import olefile" > /dev/null 2>&1 || python3 -m pip install olefile
	python3 -c "import PIL" > /dev/null 2>&1 || python3 -m pip install ".[docs]"

[radarhere]

If Pillow was already installed, but olefile wasn't, your version wouldn't install it. Unless you'd like to add olefile to the docs/Makefile install command?

[hugovk]

Yes, makes more sense to move it to docs/Makefile.