Fix Mastodon ref. impl. filename

_Originally posted by @chalin in [#9673](https://github.com/open-telemetry/opentelemetry.io/pull/9673/changes#r3176910195)_ > Rename this file to `index.md`, dropping the leading `_`, because this is a leaf page bundle, not a section. That _is_ the intention, right? cc @tiffany76 @danielgblanco