blog: use description meta as excerpt

[StefanoCecere]

if the post description meta is set, it used used instead of the full post / excerpt method

it could be a third option of

plugins:
  - blog:
      post_excerpt: description

[squidfunk]

Thanks for suggesting! Unfortunately, this is not a good idea for us to support, as it is only allowed to use plain text (not Markdown) inside the description, because it is also used for the meta description tag, as well as for social cards. No HTML, no Markdown, as it just cannot be supported. However, this can either be implemented with hooks or theme extension, i.e., custom overrides ☺️