[hook]: New hook

## Feature request
A new hook to update the content of navbar

#### What problem does this feature solve?
This will be useful as it will support the creating of dynamic navbars, control over the navbar with just markdown.

#### What does the proposed API look like?
Ref Poc https://github.com/docsifyjs/docsify/pull/1229

#### How should this be implemented in your opinion?
https://github.com/docsifyjs/docsify/pull/1229

#### Are you willing to work on this yourself?
Yeah, if anyone else wants to pick it feel free to continue

**NOTE**

This is not the priority as of now. I have opened this just for discussion as we are having some discussion as well for other features.

I would also welcome request for other new hooks as well.