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: 29 (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 every Mojo snippet in the new "Coming from Python" section against src/captions/captions.mojo and src/captions/model.mojo — all 7 API references are real, no invented APIs:

• parse_captions(source), to_srt(caps), to_vtt(caps), plain_text(caps), duration_ms(caps) — all exported from __init__.mojo and defined in captions.mojo.
• cues_between(caps, start_ms, end_ms) — signature matches exactly (cues_between(captions, start_ms, end_ms)).
• caps.cues and cue.text / cue.start_ms / cue.end_ms — real fields on Captions / Cue in model.mojo.

Python side (webvtt-py): webvtt.read(), caption iteration with .text, .start/.end, and save_as_srt() are all correct/idiomatic.

Count: "Eleven pure-Mojo libraries" checks out — 10 sibling libs listed + mojo-captions itself (unlisted, as before), consistent with the prior "Ten". mojo-xml cross-link added correctly.

No blocking issues.

[conorbronsdon]

Docs review (sonnet): SHIP

webvtt-py-comparison table verified — parse_captions, to_srt, to_vtt, plain_text, cues_between, duration_ms all real exports; Cue fields (index, start_ms, end_ms, speaker, text) match model.mojo:16-22. Correctly notes there's no Python stdlib equivalent (scopes to webvtt-py) rather than implying parity. 29-test count exact. Ship.