feat: Add OpenGraph

[Wissididom]

I also looked into oEmbed but that requires a query parameter called url that says which page is called, so I think that doesn't really make sense for a static page.
What we could think about is to render an oembed file per page with a dummy url, like /changelog.oembed.json?url=dummy or /changelog/oembed.json?url=dummy and as said that file for every site where the content changes.

The json would be something like:

{
  "version": "1.0",
  "type": "link",
  "title": "Changelog - Chatterino",
  "provider_name": "Chatterino",
  "provider_url": "https://chatterino.com"
}

in j2 maybe something like:

{
  "version": "1.0",
  "type": "link",
  "title": "{{ page_title | trim }}",
  "provider_name": "Chatterino",
  "provider_url": "https://chatterino.com"
}

where we then would need to update the title property depending on the url query parameter. If we do it with different files I think we'd need to add a link tag with a different value for each page to base.j2 and have different json files built with a different content while ignoring the url query parameter, which also seems overly complex.

This PR only implements the OpenGraph part, if we decide against adding oEmbed then this PR also solves #477

PS:

Closes #477

[Wissididom]

@jprochazk I think it would make sense if you review it as well. This is my first time working with j2 files.

[jprochazk]

https://docs.rs/minijinja/latest/minijinja/syntax/index.html#-block-

If you want to print a block multiple times, you can, however, use the special self variable and call the block with that name