Reduce per-project config: remove per-repo need to declare sbt-sonatype

[rtyley]

Currently each project that uses gha-scala-library-release-workflow needs to declare sbt-sonatype:

gha-scala-library-release-workflow/docs/configuration.md

Lines 43 to 45 in 6825045

	* [`sbt-sonatype`](https://github.com/xerial/sbt-sonatype) - you currently need to have this in your project's
	`plugins.sbt`, as the [`🎊 Create artifacts`](https://github.com/guardian/gha-scala-library-release-workflow/blob/7d278d4d44e30b4b4c0f6791053bdeb40b8159cb/.github/workflows/reusable-release.yml#L141-L158)
	job expects it, but in the future, the workflow will probably supply this automatically.

https://www.scala-sbt.org/1.x/docs/Using-Plugins.html#Global+plugins

Plugins can be installed for all your projects at once by declaring them in $HOME/.sbt/1.0/plugins/. $HOME/.sbt/1.0/plugins/ is an sbt project whose classpath is exported to all sbt build definition projects. Roughly speaking, any .sbt or .scala files in $HOME/.sbt/1.0/plugins/ behave as if they were in the project/ directory for all projects.

You can create $HOME/.sbt/1.0/plugins/build.sbt and put addSbtPlugin() expressions in there to add plugins to all your projects at once.

--sbt-dir   <path>  path to global settings/plugins directory (default: ~/.sbt)