docs: explain interpolation placeholders

[tasodoufu]

Closes #461

Problem

The existing module documentation lists interpolation namespaces but does not give contributors one consolidated reference with availability rules and an end-to-end example.

Changes

• Add a namespace/source/availability table for config, bindings, service, and secrets placeholders.
• Add a minimal cache provider/worker consumer example.
• Clarify that secrets remain Docker Compose runtime variables.

User impact

Contributors can choose the correct placeholder namespace and understand when values are resolved, without changing runtime behavior.

Verification

• python3 assertion check for all four namespaces: passed
• npx --yes markdownlint-cli@0.49.0 --config .markdownlint.json docs/modules.md: passed
• git diff --check: passed

AI-assisted contribution: the documentation draft was prepared with AI assistance and manually checked against the repository source and examples.

[RonaldHensbergen]

Thank you for your first contribution! I hope many will follow 😀
And approved 🥳