Add section about "feat." to Old style practices

https://musicbrainz.org/doc/Style/Old_style_practices is used to document things that we used to do, but are no longer doing or now doing differently.

We used to have a guideline saying that all variations of "featuring" (across languages) should be replaced with "feat.", but we have since changed this practice. However, I can't find any place under /doc/Style/ that documents the current practice and/or denounced the previous one. It would be useful to have such a place (other than the blog post) to refer to when making edits etc. (This section could even link to the blog post for additional background/detail if wanted.)