docs: Use swift-docc-plugin to preview documentation

[heckj]

Motivation

Makes it easier to preview documentation changes

Modifications

• Add dependency on swift-docc-plugin
• Add detail on how to preview into the README within the documentation target

Result

Documentation changes can be previewed by running the following command:

swift package --disable-sandbox preview-documentation --target ContainerImageBuilderPlugin

Test Plan

No functional change.

[euanh]

Thanks, this is really useful. I've been using this locally but I've been concerned that adding this dependency to Package.swift might slow down builds as reported here: swiftlang/swift-docc-plugin#68. Plugins have to be built from scratch by each project which uses them, so it's important to keep the build time as short as possible.

The good news is that local testing and the CI build times for the end to end examples suggest that adding this dependency doesn't seem to slow down projects which depend on swift-container-plugin - aside perhaps from the time needed to clone the repository in the first place. Presumably this is because swift-docc-plugin is a command plugin, so no target in swift-container-plugin depends on it, so SwiftPM decides it does not need to be built for projects which only use swift-container-plugin.

[euanh]

It's a pity --disable-sandbox is needed, but swift-docc-plugin still supports swift-tools-version: 5.7, which does not support the .allowNetworkConnections permission in Package.swift.