Prepare for version 3?

[stertooy]

• Document the recently added inputs build-needed-pkgs, build-suggested-pkgs and build-extensions
• Update action versions in README
• Validate inputs when relevant (like in run-pkg-tests)
• Made indendation consistent (2 spaces per level)

Perhaps we should also rename the CONFIGLAGS input before releasing a new version, but that can happen in a separate PR.