ci: fix docs site deployment workflow

[Maokami]

Summary

• replace leanprover/lean-action in the docs-site workflow
• install elan directly, then resolve Verso deps with lake update verso
• build the nested site/ project before generating the HTML artifacts

Why

The original Pages workflow failed on main because the nested Verso project
does not commit site/lake-manifest.json, while lean-action expects a
manifest up front for nested Lake projects.

Notes

• this does not change the root Lean project build/test workflow
• after merging, Pages should be configured with Source = GitHub Actions
• follow-up slide-quality issue: Make the Verso slide deck presentation-grade #117

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.