Update docs dependencies

[ejsmith]

Summary

• update docs dependencies to the latest available versions
• move VitePress to the latest 2.0 prerelease (2.0.0-alpha.17)
• add mermaid as an explicit docs dev dependency required by vitepress-plugin-mermaid
• refresh docs/package-lock.json

Dependency Notes

• vitepress: ^2.0.0-alpha.15 -> ^2.0.0-alpha.17
• vitepress-plugin-llms: ^1.10.0 -> ^1.12.2
• mermaid: added at ^11.15.0
• vitepress-plugin-mermaid remains current at ^2.0.17

This should supersede the open docs dependency PRs: #9, #10, #11, #18, #19, #20, and #21.

Validation

• npm audit --omit=optional reports 0 vulnerabilities
• npm outdated only reports vitepress because npm's latest dist-tag is 1.x; current/wanted is 2.0.0-alpha.17 from the next tag
• npm run build