Add social link icons to sidebar and About-me page

Small monochrome icon row (GitHub, LinkedIn, ORCID, Mastodon, Bluesky,
Matrix) rendered from a SOCIAL config tuple via a shared Jinja
partial (templates/includes/social_links.html), included unconditionally
in the sidebar below the tag cloud and again on the About-me page only.
Icons are inline SVGs (templates/icons/*.svg) using fill=currentColor,
so they track the existing accent-color/dark-mode toggle for free via
--main-link-color, no JS needed.

templates/page.html

@@ -19,6 +19,10 @@
 
     {{ page.content }}
 
+    {% if page.slug == 'about-me' %}
+    {% include "includes/social_links.html" %}
+    {% endif %}
+
     {% if page.modified %}
         <p>
         Last updated: {{ page.locale_modified }}