.nodocsify file to disable docsify from serving certain folders

## Feature request
similar to the ".nojekyll", is it possible to include a ".nodocsify" file that prevents docsify from operating on certain folders?

#### What problem does this feature solve?
adding game or software demos using iframes. Changing the github pages from "/docs" to "/root" does not work either since it messes up docsify's system for getting _sidebar.md for some reason.

#### What does the proposed API look like?
if you include a ".nodocsify" in a folder, docsify will not operate on that folder and any of its subfolders.

#### How should this be implemented in your opinion?
I'm not entirely certain, but surely it must be possible to exclude rendering if it encounters a ".nodocsify" file?

#### Are you willing to work on this yourself?
If the solution is relatively simple, perhaps. (im not very good with javascript)

Issue 中没有指定文件、测试或入口点。首先跟踪 Docsify 如何发现路由、渲染文件夹以及解析 _sidebar.md，然后确定 .nodocsify 标记会如何影响一个文件夹及其子文件夹。完成这项工作需要达成一致的行为定义，并提供覆盖被排除 demo 和普通文档文件夹的测试。

由索引模型根据 Issue 内容生成。