Allow search index starting points for additional sidebar files in nested folders

## Feature request

#### What problem does this feature solve?

Search in Docsify works by indexing the top level sidebar content. This is fine, but as a set of documentation grows there will likely be folders added, and the sidebar continues to grow. In order to limit the growth of the sidebar, we can use nested _sidebar.md files in subfolders - problem solved.

If you then check the search index via inspecting your browser's local storage, you don't see any of the content from nested _sidebar.md files until you manually navigate to them. This is because Docsify is not loading these, which is expected behavior.

The search plugin allows an `auto` mode, or you can manually list the path to _every file in your documentation_ and they'll all be indexed. For large documentation sites this is not workable.

#### What does the proposed API look like?

It would be great if we had a new configuration option for search, where we could index `sidebarPaths` (or similar). These would not need to be indexed directly, but would be used as starting points for the index since they do contain links to everything accessible in that folder.

```
search : {
paths: 'auto',
sidebarPaths : [
'/folder'
'/team-guidelines'
]
}
```

#### How should this be implemented in your opinion?

With the above config, while still working in "auto" mode, instead of only looking at `/_sidebar.md` search would additionally look at `/folder/_sidebar.md` and `/team-guidelines/_sidebar.md`.

This should honor `loadSidebar` if configured, so that people can call their sidebar whatever they like but still search it.

#### Are you willing to work on this yourself?

Yes, but I'm not a very experienced Javascript developer so I'm likely not the best fit.