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

_config.yml

@@ -30,7 +30,7 @@ masthead_title           : # overrides the website title displayed in the masthe
 # breadcrumbs            : false # true, false (default)
 words_per_minute         : 200
 comments:
-  provider               : # false (default), "disqus", "discourse", "facebook", "staticman", "staticman_v2", "utterances", "custom"
+  provider               : # false (default), "disqus", "discourse", "facebook", "staticman", "staticman_v2", "utterances", "giscus", "custom"
   disqus:
     shortname            : # https://help.disqus.com/customer/portal/articles/466208-what-s-a-shortname-
   discourse:
@@ -43,6 +43,13 @@ comments:
   utterances:
     theme                : # "github-light" (default), "github-dark"
     issue_term           : # "pathname" (default)
+  giscus:
+    repo_id              : # Shown during giscus setup at https://giscus.app
+    category_name        : # Full text name of the category
+    category_id          : # Shown during giscus setup at https://giscus.app
+    discussion_term      : # "pathname" (default), "url", "title", "og:title"
+    reactions_enabled    : # '1' for enabled (default), '0' for disabled
+    theme                : # "light" (default), "dark", "dark_dimmed", "transparent_dark", "preferred_color_scheme"
   staticman:
     branch               : # "master"
     endpoint             : # "https://{your Staticman v3 API}/v3/entry/github/"