docs: remove parameter layering docs

[awphi]

Changes

• Removes a section of documentation on parameter layering which, afaict, doesn't apply in current versions of Topo as is fairly confusing
  • With the removal of extend, this section has limited utility in normal workflows
  • It seems to suggest we parse the x-topo attribute of extended compose files which is not true - we only read parameters from the root compose.yaml

Note we still want to support extending services, but parameters are only sourced from root compose.yaml which keeps things simple. Topo Projects are defined entirely in one compose.yml or compose.yaml but can extend from other Compose projects happily. See #409 for improvements to configuring extending services.

Checklist

• 🤖 This change is covered by tests as required.
• 🤹 All required manual testing has been performed.
• 📖 All documentation updates are complete.