Add mkdocs documentation build to playground deployment workflow

[Copilot]

Extends the deploy-playground.yml workflow to build and deploy mkdocs documentation alongside the playground at /docs/.

Changes

• Python setup: Added actions/setup-python@v5 for mkdocs runtime
• mkdocs build: Install mkdocs-material==9.7.1 (pinned) and run mkdocs build
• Deploy docs: Copy built site/ directory to _site/docs/ before upload

After deployment:

• Playground remains at /
• Documentation available at /docs/

Original prompt

The github action that builds and deploys the playground should also build the documentation using mkdocs and deploy it to the same location as the playground, but under /docs/

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[Copilot]

Pull request overview

This PR extends the playground deployment workflow to build and deploy mkdocs documentation alongside the playground application.

Changes:

• Adds Python environment setup and mkdocs-material installation to the GitHub Actions workflow
• Integrates mkdocs documentation build step into the existing playground deployment process
• Configures deployment structure to serve playground at root (/) and documentation at /docs/

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[Copilot]

The documentation is being deployed to '/docs/' subdirectory, but the mkdocs.yml configuration has 'site_url: https://pro-fa.github.io/expr-eval/' (without '/docs/'). This mismatch may cause issues with internal links, asset paths, and sitemap generation in the built documentation. Consider either updating mkdocs.yml to set 'site_url: https://pro-fa.github.io/expr-eval/docs/' or using the 'use_directory_urls: false' option, or alternatively modifying this workflow step to update the site_url before building with a command like 'sed -i "s|site_url:.*|site_url: https://pro-fa.github.io/expr-eval/docs/|" mkdocs.yml' before running 'mkdocs build'.

Suggested change

	- name: Update mkdocs site_url for /docs/ deployment
	run: sed -i 's|^site_url:.*|site_url: https://pro-fa.github.io/expr-eval/docs/|' mkdocs.yml

[Copilot]

Consider adding pip caching to improve build performance, similar to how npm is cached for Node.js. The setup-python action supports caching pip dependencies by adding a 'cache' parameter. This would speed up subsequent workflow runs by avoiding re-downloading mkdocs-material and its dependencies each time.

Suggested change

	python-version: '3.x'
	python-version: '3.x'
	cache: 'pip'

[Copilot]

The 'site' directory is being copied without first verifying that the mkdocs build step successfully created it. If 'mkdocs build' fails or doesn't produce output, the copy command will fail with a potentially unclear error message. Consider adding a verification step or using a more defensive copy command like 'cp -r site _site/docs || echo "Warning: mkdocs site directory not found"' to make troubleshooting easier. Alternatively, ensure the mkdocs build command will fail the workflow if it encounters an error.

Suggested change

	cp -r site _site/docs
	if [ -d site ]; then
	cp -r site _site/docs
	else
	echo "Warning: mkdocs 'site' directory not found; skipping docs copy"
	fi