just create a folder with the same name as category in ./docs.
and write post in ./docs/{new folder}/_posts.
e.g.
ceate a category test and a post How to add a category in site with category test at a certain time.
./docs
└── test
└── _posts
└── 2019-07-15-How-to-add-a-category-in-site.md
just add a front matter.
e.g.
categories: ["category_001", "category_002"]