Add `starlight-links-validator` to documentation

[Max Kurapov]

Context

Like we had in docusaurus previously, we want to avoid adding broken links to Rafiki documentation site by checking internal links during the documentation site build step.

There is a plugin for starlight that does just that: https://github.com/HiDeoo/starlight-links-validator.

Todos

• Add https://github.com/HiDeoo/starlight-links-validator to documentation package
• Fix all current broken links