Subcategory for Jekyll Github pages

Now this blog has subcategories. Check math category page and its subcategory math/number-theory category page. The depth of subcategories can be greater than 2.

I don’t like the way I implemented this feature, so I don’t want to explain it in detail. Check the difference to see the changes.

In brief,

I defined the category structures like math/number-theory. The category key of each post can be given like category: math or category: math/number-theory, etc.
I added the directories to resemble the category structure and corresponding .md files in _categories/. For example, the category structure for now is
```
  blog
  math
     └number-theory
  miscellaneous
```
and the corresponding _categories/ structure is
```
  _categories/
            └math/
                └number-theory.md
            └blog.md
            └math.md
            └miscellaneous.md
```
(Note that the slash symbol at the end of (directory_name)/ means that it is a directory, not a file.)

Each file in _categories/ has new key parent_category whose value is all of its ancestors (from top level to lower level). For example, _categories/math/number-theory.md file has
```
  ---
  catname: "math/number-theory"
  parent_category: ["math"]
  layout: "category"
  permalink: "category/math/number-theory"
  ---
```
I modified _layouts/category.html, category.md files.
- The _layouts/category.html file shows the list of post under the category. Now,
  - it makes the hyperlinks to all of its ancestors category in heading, and
  - it shows the list of its descendant categories (if it has any).
- The category.md file shows the list of all categories of the blog. Now,
  - it shows the list of all categories with tree-like structure with indentation.

Possible issues on this implementation:

I think there are no startswith-like command for strings in Jekyll. I used contains instead; this means that I cannot make subcategory with another category name. For example, math/history-of-mathematics category will cause some problems.
_layouts/category.html file uses both of the string for category, for example math/number-theory, and the list of its ancestors, for example ["math"]. This is absurd, and the structure can be simplified.
- (2022.1.20.) Now the variable parent_category of category page is not used.

Minor update on 8th April 2024: Some urls have been updated since the blog url has been changed.