Makefile.sphinx: Add sphinx tag when building standalone package doc

Add the standalone_package_doc sphinx tag when building standalone
documentation of a swh package.

It enables to conditionnally include content in package documentation,
like indices and tables for instance.

Related to T4496

Makefile.sphinx

@@ -5,7 +5,7 @@ APIDOC_DIR = apidoc
 APIDOC_OPTS = --ext-viewcode --separate
 SPHINXBUILD = sphinx-build
 SPHINXAPIDOC = sphinx-apidoc
-SPHINX_OPTS =
+SPHINX_OPTS = -t standalone_package_doc
 SPHINX_BUILDDIR = _build
 SPHINX_SRCDIR = .