Use 'SUMMARY.md' to describe navigation structure by 'mkdocs-literate-nav' plugin

[hwhsu1231]

Suggestion

I suggest that msys2.github.io use SUMMARY.md to describe its navigation structure by mkdocs-literate-nav plugin, instead of configuring it through the nav field in mkdocs.yml.

By doing this, contributors who want to edit documentation or modify its navigation structure would only need to focus on docs_dir, which is web/.