Add giscus support (#3022)

* Add script in same style as utterances

But adjusted for the various filed differences

* Add initial script

* Add default settings

* Update changelog

* Add feature to readme

* Add comments html

* add comment provider include

* update config in docs

* Add URL for additional reference

* docs for giscus comments

* Unrelated bugfix: add missing version separator

So that things match the "history" doc.

* add space

* update history doc

* update about doc

* add to test config yaml

* remove unnecessary / incorrect async attribute

* probably should pass the right config paths

* lowercase the repo name

* Update docs to address '1' and '0' for reactions_enabled

Figured I'd match the giscus format rather than convert a boolean to an int there.

* update two additional docs

* docs wording fix

docs/_docs/18-history.md

@@ -24,6 +24,8 @@ toc: false
 - Remove `h2` from skip links navigation as it is not important for site structure. [#3012](https://github.com/mmistakes/minimal-mistakes/pull/3012)
 - Loads Font Awesome asynchronously. [#2967](https://github.com/mmistakes/minimal-mistakes/pull/2967)
 - Replace custom search icon SVG with Font Awesome icon. [#2774](https://github.com/mmistakes/minimal-mistakes/pull/2774)
+- Adds support for giscus comments. [#3022](https://github.com/mmistakes/minimal-mistakes/pull/3022)
+
 ## [4.23.0](https://github.com/mmistakes/minimal-mistakes/releases/tag/4.23.0)
 
 ### Enhancements