Excerpt guidelines

[ferdi05]

• Create guidelines to write an excerpt
• Apply them to posts without any excerpt
• Possibly rework some excerpt if needed

[ferdi05]

Will be needed for #364

[ferdi05]

We'll have to be careful with the SEO here. The excerpt of a blog post is used as the <meta name="description>" tag value.

[CaroFG]

Google best practices for creating quality meta descriptions: https://developers.google.com/search/docs/appearance/snippet#meta-descriptions

[CaroFG]

Characteristics of a good meta description: https://yoast.com/meta-descriptions/#characteristics-of-a-good-meta-description