docs: GFQL WHERE reference (restack)

[lmeyerov]

Summary

• add GFQL WHERE (same-path constraints) docs with a new reference page
• cross-link WHERE in GFQL overview/quick/predicates/spec/wire/API pages
• add brief WHERE notes in translation and Cypher mapping guides
• fold in GFQL docs catchall fixes discovered during editorial pressure testing:
  • g.gfql(ops=[...]) -> g.gfql([...]) in quick docs
  • replace bare chain([...]) snippets with runnable g.gfql([...]) forms
  • align null predicate naming to exported API (isnull/notnull, isna/notna)

Status

• Restack complete; supersedes Docs: GFQL WHERE reference #914 (closed)
• Targets master

Testing

• DOCS_FORMAT=html VALIDATE_NOTEBOOK_EXECUTION=0 ./docs/ci.sh (from docs/; Sphinx reported warnings on notebook copy-overwrite)
• RTD preview links validated (edited pages return HTTP 200)

Closes #926