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: 38 (matches README).

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 all 7 Mojo API references in the new "Coming from Python" section against real source — every one exists, no invented APIs:

• urlparse, parse_qsl, quote, unquote, urljoin, urlencode, QueryPair — all exported in src/url/__init__.mojo
• r.scheme / r.netloc / r.path / r.query — real ParseResult attributes (used in examples/parse_and_build.mojo)
• urlencode(pairs) signature confirmed List[QueryPair] (src/url/parse.mojo:468); QueryPair(key, value) takes two Strings — the "not a dict" note is accurate vs Python.
• Python-side snippets (urlparse, parse_qsl, quote/unquote, urljoin, dict-based urlencode) are idiomatic urllib.parse.

Accuracy fixes check out:

• "38 tests" matches exactly 38 test_* fns in test/test_url.mojo.
• "Eleven pure-Mojo libraries": 10 sibling bullets + mojo-url itself = 11 (consistent with the Ten→Eleven bump + one added bullet). New mojo-xml cross-link resolves.

Minor (non-blocking): the "Eleven" count relies on the reader treating mojo-url as the 11th member of its own suite — correct but implicit; a parenthetical like "(including this one)" would remove ambiguity.

[conorbronsdon]

Docs review (sonnet): SHIP

"38 tests total" verified (38 in test/test_url.mojo). Table checked against src/url/parse.mojo + model.mojo — urlparse -> ParseResult, parse_qsl, quote/unquote, urljoin, and the urlencode(List[QueryPair]) (not dict) callout all correct. Suite count consistent. No issues.