Refine social link icon spacing/sizing per feedback

Sidebar: double the standard 3rem section gap above the icon row
(6rem), horizontally centered instead of left-aligned.
About-me page: new .social-links--large variant via a
social_links_variant template var - icons at 5rem (was 1.25rem),
centered, 3rem gap from the preceding text (standard section gap,
distinct from the normal 1rem paragraph gap), wider inter-icon gap
to match the larger size.

templates/page.html

@@ -20,6 +20,7 @@
     {{ page.content }}
 
     {% if page.slug == 'about-me' %}
+    {% set social_links_variant = 'large' %}
     {% include "includes/social_links.html" %}
     {% endif %}