[MRG + 2] [MAINT] Update to Sphinx-Gallery 0.1.7 (#7986)

build_tools/circle/build_doc.sh

doc/sphinxext/sphinx_gallery/__init__.py

doc/sphinxext/sphinx_gallery/_static/gallery.css

doc/sphinxext/sphinx_gallery/backreferences.py

doc/sphinxext/sphinx_gallery/docs_resolv.py

doc/sphinxext/sphinx_gallery/downloads.py

doc/sphinxext/sphinx_gallery/gen_gallery.py

doc/sphinxext/sphinx_gallery/gen_rst.py

doc/sphinxext/sphinx_gallery/notebook.py

doc/sphinxext/sphinx_gallery/py_source_parser.py

doc/whats_new.rst