Add community card

[josh146]

Provides a custom ReST directive for creating a community card:

.. community-card::
    :title: card title
    :author: author (or subtitle?)
    :date: 30/09/2020
    :code: https://github.com/username/repo/or/notebook
    :paper: https://arxiv.org/abs/1234568
    :color: teal

    The demo description goes here. This field can contain HTML if needed, for example images and links.
    Of the options above, only title, author, and date are required. Code, paper, and color are all optional.

This will help reduce the overhead of adding community demos.

Note that this PR should result in no front-facing webpage changes.

[mariaschuld]

That will be very helpful. The card looks nice (even though not from this PR).

[glassnotes]

They look so nice! ⭐

I'm trying to test it out by adding a new block in demos_community.rst, but I cannot get anything to show up (using make html-norun). Are there any additional steps?

[josh146]

@glassnotes thanks for the help offline fixing this! The makefile should now build to the same directories in both norun and run mode

[thisac]

Didn't notice the community card that was added yesterday, but it nice that we're beginning to fill this up already. Looks great! 💯

[josh146]

Due to feedback, I've slightly modified the card design, to make the links to the paper/code more obvious: