Tooling scaffolding refresh: Dependabot, linkcheck, action-pin bumps

[mmcky]

Part of #22 — the mechanical "Bucket A" items that need no maintainer decisions.

What's in here

Change	Why
New .github/dependabot.yml (github-actions + conda, weekly, with the jupyter-book >=2.0 ignore)	The highest-value item: in the last fortnight Dependabot auto-delivered to lecture-python-advanced.myst the exact sphinx-extension bumps this repo missed. Without it this repo drifts silently.
New linkcheck.yml (weekly lychee run)	Dead links currently go undetected indefinitely. ⚠️ Note: this repo deploys Pages via the OIDC artifact workflow, so there is no gh-pages branch — the workflow mirrors the live site (dp.quantecon.org) with wget instead of the checkout-gh-pages pattern other lecture repos use.
actions/checkout v4→v6, upload-artifact v4→v7, download-artifact v4→v6	Catch up to current majors; Dependabot maintains them from here.
cache.yml: rebuild on environment.yml change (push to main)	Previously the build cache stayed stale for up to a week after an env change, so PRs built against an outdated cache.
environment.yml: sphinx-proof 0.3.0→0.4.0, sphinxcontrib-youtube 1.4.1→1.5.0, sphinx-togglebutton 0.3.2→0.4.5	Stale pins copied from an older repo's env; these are the versions the sibling lecture repos run with the same jupyter-book pin.
Remove placeholder google_analytics_id: G-XXXXXXXXXX	Non-functional placeholder; re-add with a real GA4 measurement ID if analytics are wanted.

Deliberately NOT in here

• quantecon-book-theme==0.15.1 is untouched — 0.21.0 ships a layout overhaul and needs a visual review; tracked in Bump quantecon-book-theme 0.15.1 → 0.21 (layout review needed) #23.
• PR preview deploy (Add PR preview deploy — Netlify or Cloudflare? #24) and PDF/notebook publishing (Publish PDF + downloadable notebooks? #25) — decisions pending.
• quantecon/actions/*@v0.6.0 pins — unchanged pending a QuantEcon/actions v1.0 release (tracked in Tooling: bring lecture-dp up to lecture-repo standards (tracking) #22).

Validation

• CI on this PR runs the full container build with the bumped extension pins.
• linkcheck.yml is schedule + workflow_dispatch; it can be smoke-tested from the Actions tab after merge.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR refreshes repository automation/scaffolding to reduce drift and add maintenance checks, aligning lecture-dp with other lecture repos’ tooling (Dependabot updates, scheduled link checking, and workflow pin/cache maintenance).

Changes:

• Add Dependabot configuration for GitHub Actions and conda (environment.yml) updates, with an explicit ignore for jupyter-book>=2.0.
• Add a scheduled/manual link-check workflow using lychee against a mirrored copy of the published site.
• Refresh CI/publish/cache workflows (action major bumps; rebuild cache when environment.yml changes) and update a few pinned Sphinx extension versions; remove the placeholder GA4 ID.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
lectures/_config.yml	Removes non-functional Google Analytics placeholder configuration.
environment.yml	Updates pinned Sphinx extension versions used by the build.
.github/workflows/publish.yml	Updates workflow action pins used for checkout/artifact handling during publish.
.github/workflows/linkcheck.yml	Introduces scheduled/manual link checking against the live published site.
.github/workflows/ci.yml	Updates workflow action pins used for checkout/artifact handling during PR CI.
.github/workflows/cache.yml	Triggers cache rebuilds on environment.yml changes and updates checkout action pin.
.github/dependabot.yml	Adds Dependabot updates for GitHub Actions and conda, with jupyter-book>=2.0 ignored.

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