Add missing README information to centralized docs

[dobby-coder]

Summary

Audited all 16 non-archived repo READMEs (and the postguard internal docs site) against the centralized docs pages and added missing information. This addresses the feedback in #8 that information from repo READMEs was being lost in the centralization.

Changes by repo

• pdf-signature (biggest gap — specifically called out): Added configuration parameters table with all TOML options, backend build/run commands with ROCKET_CONFIG, frontend development setup, Electron packaging instructions, and HTTP API endpoint documentation.
• cryptify: Added configuration parameters table with all TOML options, and API endpoint reference (matching the OpenAPI spec in api-description.yaml).
• postguard (second biggest gap — internal docs site had lots of content): Added cryptographic primitives section, detailed prerequisites with version numbers, per-crate build commands, Docker Compose dev environment setup, environment variables table, CLI usage examples (encrypt/decrypt), full PKG server API reference (all endpoints with request/response formats), WASM bindings API (seal, sealStream, Unsealer, StreamUnsealer), wire format specification, and Yivi integration details (sender/recipient flows, hidden policies).
• postguard-website: Added stopping services commands, Docker build commands, what services are launched, USB debugging prerequisite.
• postguard-fallback: Added IRMA attribute explanation, technical reference links, software version details, Trunk/Yew framework documentation.
• postguard-js: Added email envelope property descriptions, standalone email helpers import note, inline code comments for optional parameters.
• postguard-examples: Added pg-dotnet feature details (Encrypt & Upload vs Encrypt & Deliver), "Informatierijk notificeren" use case, project structure diagram, C# code example.
• postguard-tb-addon: Clarified build output details.

Repos with no significant gaps

The following repos had complete or near-complete centralized docs already:

• postguard-dotnet, ibe, ibs, pg-curve, pg-components, irmaseal-mail-utils, pg-example, postguard-outlook-addon (has no README)

Test plan

• Run VitePress dev server and verify all changed pages render correctly
• Check that tables, code blocks, and links display as expected
• Cross-reference a few sections against the actual repo READMEs to verify accuracy

Refs #8

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