DOC: clarify advanced doc wrt referencing examples (#806)
DOC: clarify advanced doc wrt referencing examples (#806)

* DOC: clarify advanced doc wrt referencing examples

As discussed via https://github.com/sphinx-gallery/sphinx-gallery/issues/805, the documentation could be structured a bit better to make it easier to find how to link examples. This PR suggests adding a new sub-heading *Referencing Examples* so that the term *reference* explicitly shows up in the table of contents. 

Two things to discuss:
* I'm not sure yet whether the heading should be *referencing* or *linking*. 
* Is it possible to link to heading in examples, too? Is there a special syntax for that? If yes, I could extend this PR to include it.

* DOC: add 2 projects using sphinx gallery (#807)

These are:
* https://github.com/automl/auto-sklearn
* https://github.com/openml/openml-python/

* MAINT: Fix test for links (#811)

* MAINT: Fix test for links

* FIX: Property

* Apply suggestions

* Update advanced.rst

* Update advanced.rst

* minor amendment

Co-authored-by: Eric Larson <14e6e0d1a6421c3f4a71ec06bfae870e115e40b0@gmail.com>
Co-authored-by: Lucy Liu <1e66becc72cd22c54599d523442b2f3efcfec4ab@bom.gov.au>
sphinx-gallery/sphinx-gallery,sphinx-gallery/sphinx-gallery