Support specifying short-titles for the sidebar from content pages?

## Feature request

#### What problem does this feature solve?

This feature would allow long titles for the documentation headers, with short titles in the sidebar, when needed. Right now, this can be done by manually creating each and every entry for the sidebar, but this:

a) takes *a lot* of time,
b) is very prone to errors, and
c) forces you to write each and every sidebar entry yourself, even if short-titles are only required for a couple of entries.

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

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

Right now, Docsify supports [Customise ID for headings](https://docsify.js.org/#/helpers?id=customise-id-for-headings). It would be great if it would also support something like:

```
## How would I write a "hello, world" example? :sidebar="hello world?"
```


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

Willing? yes, but realistically, I don't now how to do frontend development.

#### Relation to other issues?
This issue has of course some relation to [Proposal: auto generate sidebar](/docsifyjs/docsify/issues/1290) but as far as I can see the two are not the same, and I suspect the difficulties that would be part auto-generating the *entire* sidebar do not apply to only generating the in-page links(?)

Bắt đầu với tài liệu của Docsify về “Customise ID for headings” và so sánh đề xuất này với issue #1290 về việc tự động tạo sidebar. Theo dõi cách nội dung của heading trở thành các mục trong sidebar; được xem là hoàn tất khi hỗ trợ một tiêu đề ngắn inline như `:sidebar="hello world?"` đồng thời giữ lại tiêu đề đầy đủ của heading.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.