Improve auto-generated cards with "description" metadata

[bgravenorst]

Enhance the visibility and appeal of our auto-generated cards, such as those used in Index pages, and potentially improve the site's SEO performance. Currently, many of our pages lack a "description" metadata field, resulting in less informative auto-generated cards.

For more information, see: https://docusaurus.io/docs/markdown-features/head-metadata#markdown-page-description

[mapachurro]

I've been working on this, continuing to do so now.