Mark illustrative GFQL indexing snippets as non-runnable

[lmeyerov]

Summary

• Mark two incomplete illustrative Python snippets in docs/source/gfql/indexing.rst as skipped by the docs-example runner.
• Preserve rendered prose and GFQL behavior unchanged.

Root cause

The docs-example collector treats Python code blocks as runnable unless they carry a doc-test marker. One illustration contains a literal ...; the other references illustrative new_edges_df that is not defined in that block. This latent master issue surfaced in #1767's broader suite but is unrelated to #1767's GFQL behavior.

Validation

• Static review: one file, +4/-0; clean credential/diff checks; two consecutive no-advance review waves with zero findings.
• Guarded DGX Python 3.8.20: exact indexing.rst selector, 1 passed in 1.62s.
• Guarded DGX Python 3.14.2: exact selector, 1 passed in 2.09s.
• No project code or tests were run locally.

Follow-up

• The pre-existing docs-only CI classification gap can skip the minimal-Python lane for docs-only PRs; this narrow fix does not broaden into workflow changes.