MAINT: Add HTML archive to release assets (Phase 1)

[mmcky]

Overview

This PR implements Phase 1 of the publishing workflow upgrade as tracked in QuantEcon/meta#253.

Changes

Adds the following steps to publish.yml after the HTML build:

1. Create HTML archive - Compresses _build/html/ into lecture-datascience-html-{tag}.tar.gz
2. Generate checksum - Creates SHA256 hash for integrity verification (html-checksum.txt)
3. Create manifest - Generates metadata file with build information (html-manifest.json)
4. Upload to release - Attaches all three files to the GitHub release using softprops/action-gh-release@v2

Release Assets Created

Each publish-* tag will now include:

• 📦 lecture-datascience-html-{tag}.tar.gz - Full HTML site archive
• 🔐 html-checksum.txt - SHA256 verification file
• 📋 html-manifest.json - Build metadata (tag, commit, timestamp, size, file count)

Testing

After merging, test with a publish-test-* tag to verify:

1. Workflow completes successfully
2. Three HTML assets are attached to release
3. Archive can be downloaded and extracted
4. Checksum verification works: sha256sum -c html-checksum.txt