Document monorepo setup with the action path input

[Metbcy]

The v0.5 Action has a path: input for scanning a subdirectory, but the README only shows the root-repo case.

Scope:

• Add a short monorepo example to docs/src/github-action.md.
• Mention when to use path: services/api versus bring-your-own SBOMs.
• Link from the README quick links or Detailed install section if it stays concise.

Definition of done:

• mdbook build docs succeeds.
• The example remains copy-pasteable and does not reintroduce manual Syft wiring.