Set meta description per-page

Currently, the description meta tag is set to a site-wide value, on every single exported HTML page:

```

```

We need a proper couple of sentences, different for each page, to actually showcase what the page is about.

A couple of possible solutions:
- Set description manually, per-page. This is the best as we can prepare curated "extracts". I'd like to be able to set it per-markdown file, maybe in a YAML front matter. See https://docs.rs/mdbook-frontmatter/0.0.4/mdbook_frontmatter/ can be useful
- Unstrip the `{{{ content }}}` value of the HTML, ignore the headers and consider the first N (30, 40?) characters. Maybe even try to detect the first M (2, 3?) sentences.

Example of a Twitter generated card for a page in Pan Docs after this modifications: