docs(pg-manual): explain low-level WASM example and IRMA demo flow

[dobby-coder]

What

Rewrites the six-line pg-manual/README.md stub into a full README, matching the depth of the other sub-project READMEs (pg-sveltekit, pg-node, pg-dotnet).

The new README covers:

• What pg-manual is: a browser example driving PostGuard via the low-level @e4a/pg-wasm library directly (seal/Unsealer/sealStream/StreamUnsealer), in contrast to the high-level @e4a/pg-js SDK used by pg-sveltekit and pg-node.
• The two entry points (string.html vs file.html) and what each demonstrates.
• The key flow: PKG parameter fetch, Yivi popup for attribute disclosure, key retrieval, seal/unseal.
• IRMA demo credentials: the irma-demo.* attributes the example uses and where to issue demo instances from the attribute index.
• Prerequisites, run steps (dev server on http://localhost:9000), and a note on the PKG_URL constant in examples/utils.js (currently the legacy iHub deployment, see pg-manual: PKG_URL still points at legacy iHub deployment (main.postguard.ihub.ru.nl) — intentional? #54).
• When to reach for pg-manual vs the SDK-based examples.

Docs only. No code changes.

Closes #53

🤖 Generated with Claude Code