docs: add Coming-from-Python section + fix stale counts

[conorbronsdon]

Summary

Two documentation changes:

1. Coming from Python — a short side-by-side section (near the top, after
   the intro) mapping the familiar Python API to this library's real API.
   Every snippet was extracted from this repo's own examples/ and test/;
   nothing was invented.
2. Accuracy fixes — Suite copy Ten -> Eleven and added mojo-xml link. Test count re-verified by counting: 23 unit tests (matches README; conformance rows unchanged).

Test counts were re-verified by actually counting the test functions in the
repo (not trusting the prior text).

From an automated review (personal-context#62); drafted by Claude Code — please review the Coming-from-Python snippets for API accuracy.

[conorbronsdon]

🤖 Independent Claude review: Ready.

Verified every Mojo snippet against src/unicodedata/normalize.mojo — no invented APIs:

• Table: normalize(form, s) (L208), is_normalized(form, s) (L224), casefold(s) (L218) — all real module-level fns, all exported in __init__.mojo.
• Prose: Normalizer() + .normalize(form, s) / .casefold(s) — real (struct L66, __init__ L71, normalize L174, casefold L189).

Other checks:

• Python side is idiomatic: unicodedata.normalize, unicodedata.is_normalized (3.8+), and pairing str.casefold() → casefold(s) is correct (Python's unicodedata has no casefold).
• "Eleven libraries": 10 listed siblings + mojo-unicodedata itself (not self-listed) = 11 — consistent with the prior "Ten"/9-bullet convention.
• New mojo-xml cross-link resolves (repo exists, public).

No wrong counts, no non-existent APIs.

[conorbronsdon]

Docs review (sonnet): SHIP

Coming-from-Python table (normalize("NFC", s), is_normalized(...), casefold(s), Normalizer() reuse) matches src/unicodedata/normalize.mojo signatures exactly. 23-test count verified. Suite Ten→Eleven arithmetic correct. No issues.