docs: add demo validation to the 0.5 release checklist

[codeforester]

Fixes #434. Stacked on #432. Adds exact-candidate base-cli-demo validation and the post-publication demo #47 sequence. Validation: scripts/validate_docs.py.

[codeforester]

Review: docs-only and consistent with #436. Two things to fix.

1. Step 4 says to run the pinned base-cli-demo revision against the candidate wheel. The demo declares base-cli>=0.4.3,<0.5 (see base-cli-demo pyproject), so a normal pip install of the demo would either fail resolution against 0.5.0 or replace the candidate with 0.4.x. Say to install the demo with --no-deps after the candidate wheel and run pip check/verify the installed version, as ci: verify downstream fixtures use the release candidate #444 does for the fixtures. Otherwise the check can pass against the wrong framework.
2. Ordering: step 7 says to close the tracker once everything is verified, and the new step 8 (demo#47 consumer validation) comes after it and gates completion. Either move step 8 before the close or reword step 7 so the tracker stays open until step 8.

Not ready to merge until it is rebased onto #436 anyway (stacked).