update link fixer so it does't append .md in links

If the live site doesn't, then the generator doesn't. We may need to update the [link fixer](https://github.com/cncf/contribute-site/blob/main/.github/workflows/link-fixer.md) workflow to not do this. The bots are a work in progress :)

_Originally posted by @nate-double-u in https://github.com/cncf/contribute-site/pull/266#discussion_r2943850537_
            