Unify the sidebar / markdown headings in cotent / TOC render logic

[Timeless0911]

          If we use `dangerouslySetInnerHTML`, it forces users to write HTML entities `&lt;` instead of `\<` which is more common in mdx writing. And the sidebar / markdown headings in cotent / TOC render logic should be unified.

Originally posted by @Timeless0911 in #1557 (comment)