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. Corrected test count 24 -> 29 in both README and CHANGELOG (re-verified by counting test functions: 29).

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 diff against src/tar/ — all APIs are real, no inventions:

• read_tar_file(path) → src/tar/tar.mojo (returns List[TarEntry])
• e.info.name / e.info.size / e.info.mode / e.info.mtime / e.info.linkname → TarEntry.info + TarInfo fields in src/tar/model.mojo
• e.info.isfile() / isdir() / issym() / islnk() → all defined on TarInfo (model.mojo)
• TarWriter() no-arg ctor → tar.mojo
• w.add(name, data, mode=0o644, mtime=0) → matches TarWriter.add signature exactly (mode/mtime are real kwargs)
• w.finalize() returns List[UInt8] → confirmed
• add_dir(name) / add_symlink(name, target) → both present on TarWriter

Counts checked:

• 29 tests — matches exactly (29 test_* fns in test/test_tar.mojo); CHANGELOG + README both updated. ✓
• Eleven libraries — 10 sibling libs listed + mojo-tar itself = 11, consistent with the prior "Ten". ✓

Python-side snippets (tarfile.open, getmembers(), m.name/.size, m.isdir()/.issym()) are idiomatic and correct.

Minor (non-blocking): the tf.add(...) → w.add("f.txt", data, ...) row is a loose analogy — Python's tarfile.add() reads a path from disk while mojo-tar's add takes in-memory bytes. The "/ write member" label makes the intent clear, so fine as a conceptual map.

No invented APIs, no wrong counts.

[conorbronsdon]

Docs review (sonnet): SHIP

Test count 24→29 verified exact (grep -c "^def test_" = 29). "Coming from Python" table checked line-by-line against src/tar/tar.mojo + model.mojo — all signatures match (read_tar_file, TarWriter().add(...), .finalize() -> List[UInt8], TarInfo.isfile/...). Suite count Ten→Eleven arithmetic correct; mojo-xml link resolves. CHANGELOG count consistent. No issues.