Add sphinx-gallery cross ref instructions to documenting guide

[story645]

Problem

The section in the documenting docs on cross referencing includes instructions for sphinx, but not for sphinx gallery.

Suggested Improvement

Add a sentence like "examples and tutorials are .py files rendered into rst using the sphinx-gallery plugin. The pattern for cross referencing these documents is :ref:sphx_glr_<folder>_<pyfile>`, for example:

:ref:`sphx_glr_tutorials_provisional_mosaic.py`

More info at https://sphinx-gallery.github.io/stable/auto_examples/plot_0_sin.html?highlight=cross%20reference#cross-referencing

[anntzer]

Actually using

:doc:`/gallery/foo/bar`

is better, as that's not s-g specific but works basically always.

[story645]

🤷 seemed to be creating problems on this weeks GSOD call?
attn: @tacaswell

[timhoffm]

I think :doc: would be fine. IIRC Jerome tried to reference the heading of the example/tutorial using :ref:.