feat: Document support image preview

[Jimmylxue]

About PR

Thanks for checking

We often insert pictures in documents，But currently does not support preview images, so i add this feature on this pr.

Please forgive me and give pointers if there is any bad writing.

we can preview like this:

Operation Video

demo

As long as img is used in the document, clicking on this image can trigger the preview effect， like this:

![image-20220103230652865](https://vitepress-source.oss-cn-beijing.aliyuncs.com/typoraimage-20220103230652865.png)

![An image](/assets/bg.jpg)

Thanks again

[kiaking]

Could be nice feature to add, but I would like to adjust the stylings a bit. I'll look into it when I get time!

[brc-dd]

Also, there was some discussion on this. Refer #854, it is straightforward to add if someone wants (the example is outdated though 😅, https://github.com/vuejs/vitepress/issues/854#issuecomment-1233866453 will fix route change issues).

[Jimmylxue]

would thanks, I am looking forward to updating as soon as possible！

[Charles7c]

Is there still a chance for this feature?