Improve OpenGraph snippets

[jsha]

Per https://meta.discourse.org/t/improving-onebox-for-our-documentation-pages/185679/2, I'd like to improve our OpenGraph snippets. My primary goal is for documentation pages to be displayed really nicely when they are linked in the forum, and include a good amount of text. My secondary goal is for pages to be displayed better on social media.

In particular I'd like to get rid of the boilerplate "Last updated: Jul 30, 2019 | See all Documentation"

[tdelmas]

In short-term, filling the description attribute in the top should work, but it's manual...

[tdelmas]

With the macro {{< lastmod >}} replaced by an attribute show_lastmod: 1, the boilerplate "Last updated: Jul 30, 2019 | See all Documentation" is no longer part of the description: #1240