include more metadata in docs pages

[elstoc]

It would be nice to include some metadata so that this data populates correctly when linking from external sites.

Some googling suggests that LinkedIn (for example) needs the following:

<meta property='og:title' content='Title of the article"/>
<meta property='og:image' content='//media.example.com/ 1234567.jpg"/>
<meta property='og:description' content='Description that will show in the preview"/>
<meta property='og:url' content='//www.example.com/URL of the article" />

This link to stackoverflow has some useful suggestions for various other sites.

Given that this is ultimately hosted on darktable.org, we should probably use similar information as the main darktable.org site, for which I've raised a separate issue.