Allow use category id as heading's id

[YDX-2147483647]

What kind of change does this PR introduce?

• Bugfix
• New Feature
• Feature Improvement
• Refactoring
• Documentation
• Other, please describe:

Description:

This PR add a configuration category_heading. It determines how categories headings are generated.

• If simple, headings will be ## Category, and IDs are set by GitHub. (Same as current version)
• If robust, headings will be <h2 id='category-id'>Category</h2>.

Resolves #65

TOC relies on these IDs. If all of categories' names are ASCII, simple is OK. Otherwise, GitHub may slugify the title into something unexpected (e.g. 🌎Globe → Globe but ⚒️Hammer → %EF%B8%8FHammer), and links in TOC will be broken.

I've also introduced an (internal & unused) BREAKING change: title_md_prefix: str = "##" → heading_level: int = 2. As you can see, title_md_prefix can't describe <h2>.

Checklist:

• I have read the CONTRIBUTING document.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[LukasMasuch]

LGTM 👍