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 — No accuracy fix needed: suite copy already read "Eleven" and already listed mojo-xml. Test count re-verified by counting: 70 (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 6 Mojo snippets in the Coming-from-Python table check out against src/ and examples/:

• parse_feed(text) → src/feed/feed.mojo (def parse_feed(var source: String, ...)), used in examples/list_episodes.mojo
• fetch_feed(url) → src/feed/fetch.mojo (def fetch_feed(url: String))
• feed.title, feed.items → Feed struct fields in src/feed/model.mojo
• item.title, item.pub_date → FeedItem struct fields in src/feed/model.mojo (note: field is pub_date, correctly matched to feedparser's entry.published)

No invented APIs. Python-side feedparser mapping is idiomatic (parse(text|url), d.feed.title, d.entries, entry.title/entry.published).

Count re-verified by counting test fns via API: test_date 11 + test_feed 19 + test_real_feed 10 + test_xml_parser 30 = 70, matching README ('70 tests across four files … XML tokenizer (30)'). 'Eleven pure-Mojo libraries' and the mojo-xml cross-links are present. The diff itself only adds the table (the accuracy items were already correct upstream, as the PR body notes).

[conorbronsdon]

Docs review (sonnet): SHIP-WITH-NITS

The 5-row feedparser→mojo-feed table is accurate (parse_feed, fetch_feed, Feed.title/.items, FeedItem.title/.pub_date all real fields, correctly shown as attribute access). 70-test count exact. Suite count was already correct pre-PR.

Nit: the title/description says "fix stale counts," but nothing was stale — the diff is a pure +12/-0 addition with zero count changes. Title overstates what the PR does; trim before merge (harmless).