sbt warning about unused setting `docs / previewSite / previewPath`

Running sbt shows the following warning each time:
```
[warn] there's a key that's not used by any other settings/tasks:
[warn]
[warn] * docs / previewSite / previewPath
[warn] +- /home/runner/work/pekko-persistence-r2dbc/pekko-persistence-r2dbc/build.sbt:94
[warn]
[warn] note: a setting might still be used by a command; to exclude a key from this `lintUnused` check
[warn] either append it to `Global / excludeLintKeys` or call .withRank(KeyRanks.Invisible) on the key
```
I think this is because about a year ago `SitePreviewPlugin` was [changed](https://github.com/sbt/sbt-site/blame/601922e00a77753d950a0b7afce83f514fafd180/core/src/main/scala/com/typesafe/sbt/site/SitePreviewPlugin.scala#L10) to no longer automatically enable on modules that have other `sbt-site` plugins added.
Would it be okay to add `SitePreviewPlugin` to `docs` module, or should we remove the setting? Seems to me that having the plugin enabled would be helpful, or at the very least harmless.

Start at build.sbt:94 and inspect the docs module's SitePreviewPlugin configuration. Compare whether enabling SitePreviewPlugin or removing docs / previewSite / previewPath matches the current sbt-site behavior. Run sbt and confirm the unused-setting warning is resolved.

Written by the indexing model from the issue text.