Prevent broken links in `dstack` docs

[jvstme]

dstack docs often contain broken links that hurt the user experience. To prevent broken links, these measures are suggested:

• Build docs in strict mode (mkdocs build -s) so that pipelines fail if there are broken links
• Build docs in pull request pipelines (without publishing or with publishing to a staging environment) to prevent merging changes that break links
• In doc files, only use local links and avoid using full self-referencing URLs (https://dstack.ai/docs/...)
• See if building in strict mode fails when there are broken anchor links. If not, come up with a solution
• See if there are solutions to check external links when building

[github-actions[bot]]

This issue is stale because it has been open for 30 days with no activity.