Use `parallel_build_count` in plugins and document usage

[mr-cal]

What needs to get done

• Ensure all plugins use ProjectInfo.parallel_build_count if possible
• Update plugin documentation to mention if the plugin supports parallel builds

The rust and scons plugins support parallel builds but do not use it. Other plugins need to be checked.

Why it needs to get done

To speed up build times or to limit resource utilization of a build

source: https://github.com/canonical/craft-parts/issues/631#issuecomment-1908500081

[syncronize-issues-to-jira[bot]]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3099.

This message was autogenerated