👌 IMPROVE: Add link-alt to fix card link accessibility

[chrisjsewell]

This PR adds the link-alt option to card (and grid-item-card) directives,
in order to assign a discernable name to the link (for screen readers).

As noted in #78, it may be ideal to add aria-label/aria-hiddento the actual linkHTML element. However, this would entail having to override aspects of the sphinx HTML builder. Instead, we include the link alt text, but add asd-hide-link-textCSS class, to setfont-size: 0`, to hide the text.
This solution was taken from https://stackoverflow.com/questions/471510/hide-text-using-css, and seemed to be the simplest solution.

fixes #78

[chrisjsewell]

@bertiewooster what do you think of this solution?

[bertiewooster]

Thanks @chrisjsewell! Yes, from what I can tell, that seems to solve the problem. I checked by comparing the Sphinx design Cards page canonical version to the version built from this pull request. It would be best if someone who uses a screenreader could verify that this solution works for their reader.

Before

Lighthouse report includes Links do not have a discernible name

<a class="sd-stretched-link reference external" href="https://example.com"></a>

After

Lighthouse report does not contain that problem

<a class="sd-stretched-link sd-hide-link-text reference external" href="https://example.com"><span>example</span></a>