Enable tiered category hierarchy/dropdowns for docs

[Zijian-Zhu]

I create a directory named overview-example-level-1-dir under docs/en to make an example of this tiered docs.

The following screenshot is the example of the actually docs and directory under docs/en/overview-example-level-1-dir

As you can see overview.md and Getting-Started.md are level 1 docs. Architecture.md and FAQ.md are level 2 docs.
JobService.md is level 3 docs and Use-Cases.md is level 4 docs.

The following screenshot is example of content in the new Yaml file.

All the sub-directories need to be listed in the subitems with their title which does not need to be the same as their directory name and their buttonTitle which is the title without white space since white space will bug in the HTML.
All the docs located in each directory need to be listed in the subfiles with docs title and url which is the path of the doc after jekyll bulid (this path always the same of the path of the files except the file type is .html not .md

The following screenshot is the output of the example.

This is the full expansion of the example list. The blue string is the URL of the corresponding docs. And the button is collapse type and use the buttonTitle as name

[LuQQiu]

@Zijian-Zhu Thanks for the docs structure!
The levels looks good!

Some small suggestion about the format style:
Ideally directory and docs on the same level have the same visual effect, or slightly different visual effect
for example, level-2-dir matches the Overview and getting started style, or could have a different color indicating it's a folder instead of a doc.

[Xenorith]

need to fix the doc check. it used to ensure the categories on each page is an existing category listed in the config yml, but now that mechanism is removed, so we could also remove the check.

in a future PR, we should add back a similar check to ensure that config yml file list and the list of all doc md files are exactly the same.