bring back repo_id and category_id in giscus section (alshedivat#1892)

After deploying my website (Cloudflare Pages), my Giscus did not work, I
realized these two paramesters are removed. After adding them again, my
Giscus worked correctly.
These new values are values from previous commits of original repository

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Co-authored-by: Maruan Al-Shedivat <[email protected]>

Author: alerezaaa

Diff for: _config.yml

@@ -137,7 +137,9 @@ related_blog_posts:
 # the information below.
 giscus:
   repo:                          # <your-github-user-name>/<your-github-repo-name>
+  repo_id:                       # leave empty or specify your repo_id (see https://giscus.app/) 
   category: Comments             # name of the category under which discussions will be created
+  category_id:                   # leave empty or specify your category_id (see https://giscus.app/) 
   mapping: title                 # identify discussions by post title
   strict: 1                      # use strict identification mode
   reactions_enabled: 1           # enable (1) or disable (0) emoji reactions