Generate useful description for OpenGraph/oEmbed links

When one-boxing a link to the new website on the forum, e.g. in [this post](https://forum.image.sc/t/imagej-skeletonize3d-plugin-download/54026/4?u=imagejan), the description currently reads:

> ImageJ is an open source image processing program designed for scientific multidimensional images.

It's possible to specify the description with `description:` in the front matter, but ideally we should improve the default description to be the same that is also used in the search (see [this Gitter discussion](https://gitter.im/imagej/imagej.github.io?at=60ca3eb4b346e9618bd5b6f7)).

@ctrueden suggests translating the Python code here:

https://github.com/imagej/imagej.github.io/blob/2f99613adf54323d6f8705d5c7a32f37180c52a0/_search/server/index-site.py#L18-L32

... to Liquid and call it from here:

https://github.com/imagej/imagej.github.io/blob/2f99613adf54323d6f8705d5c7a32f37180c52a0/_includes/layout/onebox-info#L25-L27

Compare the description logic in _search/server/index-site.py at lines 18-32 with the Liquid include at _includes/layout/onebox-info lines 25-27. Start by reading those two locations and the linked discussion, then verify that links without an explicit front-matter description produce the same default description used by search while explicit descriptions remain supported.

Written by the indexing model from the issue text.