QEP-1 v4: codify the in-document appendix

QEP-5 is the second QEP to carry a normative machine-readable appendix — its Code in brief block, bounded by <!-- qep-appendix: code-in-brief --> and <!-- /qep-appendix --> — after QEP-2's co-located yml. Two instances was the agreed trigger for generalising the convention into QEP-1. QuantEcon/cli#50 consumes QEP-5's block by substring extraction, so the convention has a consumer now.

Proposed shape, to be settled on the amendment pull request:

• an optional, normative block bounded by <!-- qep-appendix: <id> --> and <!-- /qep-appendix -->, extractable as a substring with no markdown parsing;
• any change inside the block is a substantive amendment and bumps version;
• the QEP names the tool that consumes the block;
• everything stays in the one markdown file: a fenced block for data, a GitHub alert for prose;
• decide whether QEP-2's yml migrates into a fenced block or stays as the one file-form exception. The check in check-labels.mjs assumes the file form today.

This is a substantive amendment to QEP-1 (v3 → v4) and needs a second reader. Lands after the rollout has exercised the block at least once, so the convention is written from use.

Read QEP-1, QEP-2, and QEP-5 to compare the existing appendix forms and review check-labels.mjs, which assumes QEP-2's current file form. Follow the rollout using QuantEcon/cli#50, then settle the proposed QEP-1 v4 convention in an amendment pull request with a second reader.