Add navbar instructions and add to main and quickstart and maybe nested.

[MichaelCurrin]

Including use of multiple navbars and try this out. It may not be that useful though.

https://docsify.now.sh/custom-navbar

_navbar.md is loaded from each level directory. If the current directory doesn't have _navbar.md, it will fall back to the parent directory. If, for example, the current path is /guide/quick-start, the _navbar.md will be loaded from /guide/_navbar.md.