*.markdown: rename to *.md

[classabbyamp]

does not affect any content, it just annoys me

[classabbyamp]

can confirm it builds fine and there is no difference in the site:

riff /tmp/void-web-master/_site /tmp/void-annoying/_site
diff -ur --show-c-function --new-file /tmp/void-web-master/_site/atom.xml /tmp/void-annoying/_site/atom.xml
--- /tmp/void-web-master/_site/atom.xml 2024-02-12 23:36:37.332504264 -0500
+++ /tmp/void-annoying/_site/atom.xml   2024-02-12 23:36:28.743462157 -0500
@@ -5,7 +5,7 @@
        <subtitle>Follow Void Linux news</subtitle>
        <link href="https://voidlinux.org/"/>
        <link type="application/atom+xml" rel="self" href="https://voidlinux.org/atom.xml"/>
-       <updated>2024-02-12T23:36:35-05:00</updated>
+       <updated>2024-02-12T23:36:27-05:00</updated>
        <author>
                <name>Void Linux Team</name>
        </author>

[Vaelatern]

I'm happy with this.