Add support for webmentions

[chringel21]

I like the concept of webmentions and would like to add support for it in this theme.

There are a couple of resources with notes on implementations for webmentions and Hugo (i.e. here, here, here, here). They involve webmention.io and a self-hosted solution using go-jamming. I would like to add support for both.

Displaying webmentions can be achieved using either webmention.js or a custom implementation by wgroenveld

• Add support for webmention.io (default option for webmentions)
• Add support for self-hosted solution go-jamming
• Add rendering of webmentions in templates using webmentions.js (default option)
• Add rendering of webmentions in templates using a custom implementation (i.e. https://github.com/wgroeneveld/jam-my-stack/blob/main/src/webmention/get.js)