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/icons/orcid.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" role="img" aria-hidden="true" focusable="false"><path fill="currentColor" d="M12 0C5.372 0 0 5.372 0 12s5.372 12 12 12 12-5.372 12-12S18.628 0 12 0zM7.369 4.378c.525 0 .947.431.947.947 0 .525-.422.947-.947.947-.525 0-.946-.422-.946-.947 0-.516.421-.947.946-.947zm-.722 3.038h1.444v10.041H6.647V7.416zm3.562 0h3.9c3.712 0 5.344 2.653 5.344 5.025 0 2.578-2.016 5.025-5.325 5.025h-3.919V7.416zm1.444 1.303v7.444h2.297c3.272 0 4.022-2.484 4.022-3.722 0-2.016-1.284-3.722-4.097-3.722h-2.222z"/></svg>