Add Mastodon icon (#4)

Author: Armavica

src/pymc_sphinx_theme/components/pymc_icon_links.html

@@ -1,6 +1,7 @@
 {%- if not theme_icon_links -%}
   {%- set theme_icon_links = [
     { "url": "https://github.com/pymc-devs", "icon": "fa-brands fa-github", "name": "GitHub" },
+    { "url": "https://bayes.club/@pymc", "icon": "fa-brands fa-mastodon", "name": "Mastodon" },
     { "url": "https://twitter.com/pymc_devs/", "icon": "fa-brands fa-twitter", "name": "Twitter" },
     { "url": "https://www.youtube.com/c/PyMCDevelopers", "icon": "fa-brands fa-youtube", "name": "YouTube" },
     { "url": "https://discourse.pymc.io", "icon": "fa-brands fa-discourse", "name": "Discourse" },