Replace Example Gallery with Example Index

[kelle]

• incorporate all examples into the documentation
• add tag to examples so they can be indexed
• create Example Index

[sosey]

@kelle @eteq You could put a link in the sphinx docs directly to a notebook with the example you want to show, this will pop it up in colab, which might be an interesting option for people:

https://colab.research.google.com/github/astropy/astropy-workshop

More info:
https://colab.research.google.com/github/googlecolab/colabtools/blob/master/notebooks/colab-github-demo.ipynb

[eteq]

It turns out #7242 is a duplicate of this. @kelle indicated we should close this one since #7242 has a bit more detail. So I will do that. But I like @sosey's suggestion so will cross-post it there!