docs: unsmash JSON-LD and Cypher export pages

[Frankie-Xu]

Summary

• Split the hand-merged ## he export jsonld / ## he export cypher sections in docs/en/cli/commands/export.md and docs/zh/cli/commands/export.md so each format has its own description, synopsis, options, and examples.
• Merge the contradictory Chinese format lists into one line matching English: obsidian, graphml, jsonld, cypher, csv.
• Fix the Python API example into two valid export_to_* calls and mention Cypher in the intro.

Closes #150

Out of scope

• Rewriting GraphML / CSV / Obsidian sections
• Encoder or CLI changes
• Switching documented Python helpers from export_to_* to export_ka_*

Test plan

• Adjacent jsonld/cypher headings now have a section break
• ast.parse of the fenced Python blocks in both files
• jsonld section has no he export cypher; cypher section has no he export jsonld

[yifanfeng97]

Merged — same merge fallout family as #149 (my keep-both resolution smashed the two sections together). Verified the spacing is restored.