Keep development prerequisites tied to repository manifests

[vitormattos]

Summary

Remove stale hard-coded PHP/Node/npm versions from the Developer Manual and make the current LibreSign manifests/CI the source of truth.

Changes

• direct contributors to composer.json for the PHP platform requirement;
• direct contributors to package.json#engines for Node/npm;
• avoid copying transient runtime versions into durable documentation;
• align the NCDD setup note with the current per-project *.localhost HTTPS proxy model instead of assuming http://localhost and a fixed container name.

This was discovered while validating the Developer Manual as the canonical destination referenced by the new operational AGENTS.md.

Validation

• commit created through the signed Git MCP;
• DCO trailer matches the configured identity;
• GitHub reports the OpenPGP signature as verified;
• documentation CI will build the complete tree with warnings as errors.