Introduce a "description" field to the frontmatter of posts

[mansona]

This would allow people to specify the short version of a post that could be used in the PostCard component (in the index) and in the SEO description field.

There are some design aspects that need to be ironed out on exactly how this would work but it should be implemented in an optional way, i.e. if you don't provide an explicit description you are given the "excerpt" implementation that currently works.