Zettel preview: adding graph metadata

[GenrihGrigoryan]

It would be nice, to have possibility to choose the default preview of generated Neuron site, and to customise it for each page, if needed.
It can be done by adding, properties in frontmatter. Fo example: 'preview-image: [image]' - for image, 'preview-description: [text]' - for for description.

Main goal - to achive preview of page in social media (facebook, telegram, twitter, etc) from this:

To something like this:

Possible solution - Open Graph Protocol: https://ogp.me

[GenrihGrigoryan]

update:

It can be done, by adding these lines of code in header.html:

<meta property="og:image" content="static/black.png">
<meta property="twitter:image" content="static/black.png">

Then, the better idea: is possibility to automatically generate image preview with title of black, or white background.
For example, i'm choosing preview background in "neuron.dhall":

use preview-image = True,
preview-background = black

And after sharing a link, people will see something like this:

[srid]

Neuron already uses Open Graph Protocol. It will use the first paragraph as description, and the first image (if any) as the preview image. However we can make them overridable in the YAML front matter. It would be a good first contribution.

EDIT: Yea, sitewide OGP configuration can be specified in neuron.dhall. RFC and PR welcome.