docs: fixup changelog

Give the `main` page a non-empty title in the TOC.
The intent is that we'll roll that page over to a release specific
page when we cut a release and then empty out `main` again.

Enable the magiclink extension for mkdocs, so that GH issue number
links work without any additional ceremony.

docs/changelog/main.md

@@ -1,2 +1,4 @@
+# Unreleased changes on the mainline
 
 * Expose ready queue size to metrics. #30
+* Fixed IPv6 lookups for domains without MX records

mkdocs-base.yml

@@ -63,6 +63,10 @@ markdown_extensions:
   - admonition
   - pymdownx.details
   - def_list
+  - pymdownx.magiclink:
+      repo_url_shorthand: true
+      user: KumoCorp
+      repo: kumomta
   - pymdownx.highlight:
       anchor_linenums: true
       line_spans: __span