Document conditional put support as part of S3 API compat

[Frederik-Baetens]

AWS implemented this for PutObject after we did, which is why it was in the extensions section. Now that AWS has implemented this, we can move it into the regular S3 api document, instead of the extensions document.

Summary

S3 implements this now, so we can just check it off as a standard operation.

Documentation checklist

• [] Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.

No changelog because this is a maintenance change, I don't think anyone would like to be notified of this documentation change.

• The change adheres to the documentation style guide.