Add changelog entry for SourceCode functionality

Author: jb3

docs/changelog.rst

@@ -7,6 +7,7 @@ Changelog
 .. XXX: CHANGE DATE BEFORE RELEASE
 
 - :release:`12.0.0 <9th November 2025>`
+- :feature:`310` Provide a pre-built :obj:`pydis_core.exts.source.SourceCode` cog for providing links to command implementations.
 - :support:`309` Dependency bumps on all dependencies
 - :support:`309` Migrate build system from Poetry to uv
 - :support:`309` Explicit support for Python 3.13 and 3.14