docs: catch up once the v0.12.0 follow-ups merge (canary preview-cloudflare, cname removals)

[mmcky]

docs/dev/PLAN.md:35-38 records two follow-ups from v0.12.0 that live outside this repo. Both now have PRs; once they merge, this repo's docs need a small catch-up.

When QuantEcon/test-actions-lecture-intro#67 merges (a permanent preview-cloudflare@v0 step deploying to qe-preview-canary; its CI was green and posted a pr-67 alias):

• CONTRIBUTING.md, Staged Releases: remove the known limit at line 99 ("preview-cloudflare is not yet in the canary's ci.yml …"), and make step 2 (line 88) say the canary PR runs real preview-netlify and preview-cloudflare deploys.
• docs/dev/PLAN.md: the test-actions-lecture-intro row of the consumers table (line 25) gains preview-cloudflare; remove the matching follow-up bullet (line 38).
• CHANGELOG.md: an [Unreleased] Documentation line.

When the six cname removals merge (QuantEcon/lecture-python.myst#1064, QuantEcon/lecture-jax#350, QuantEcon/lecture-python-intro#847, QuantEcon/lecture-python-advanced.myst#385, QuantEcon/lecture-python-programming#636, QuantEcon/continuous_time_mcs#236):

• docs/dev/PLAN.md: remove the cname follow-up bullet (line 37), and with it the "Follow-ups from v0.12.0" heading once both bullets are gone.

The line numbers are from main on 2026-09-25.

Assisted-by: Claude Code (Claude Opus 5.5)