social: add mastodon icons in footer

So people can find out more about MPTCP and/or me.

Icons from Wikipedia, under the terms of the GNU Affero General Public
License:

  https://commons.wikimedia.org/wiki/File:Mastodon_Logotype_(Simple).svg

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

_config.yml

@@ -52,11 +52,20 @@ minima:
 
   # Generate social links in footer.
   social_links:
+    - platform: mastodon
+      user_url: "https://social.kernel.org/mptcp"
+    - platform: github
+      user_url: "https://github.com/multipath-tcp"
+    - platform: mastodon-colours
+      user_url: "https://fosstodon.org/@matttbe"
+    - platform: github
+      user_url: "https://github.com/matttbe"
+
   #  - { platform: devto,          user_url: "https://dev.to/jekyll" }
   #  - { platform: dribbble,       user_url: "https://dribbble.com/jekyll" }
   #  - { platform: facebook,       user_url: "https://www.facebook.com/jekyll" }
   #  - { platform: flickr,         user_url: "https://www.flickr.com/photos/jekyll" }
-    - { platform: github,         user_url: "https://github.com/multipath-tcp" }
+  #  - { platform: github,         user_url: "https://github.com/jekyll" }
   #  - { platform: google_scholar, user_url: "https://scholar.google.com/citations?user=qc6CJjYAAAAJ" }
   #  - { platform: instagram,      user_url: "https://www.instagram.com/jekyll" }
   #  - { platform: keybase,        user_url: "https://keybase.io/jekyll" }