Section highlighting does not yet support all adornment characters

[lextm]

http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#sections

The standard defines a series of feasible characters, but not all of them are properly highlighted (such as ^ and :).

[lextm]

Due to implementation complexity and typical usage of the characters, I don't think all of the sphinx/docutils supported ones should be supported by this extension.

However, the most common ones should be supported, in case they exist already in tons of documentation projects,

• (if we choose a minimal) 6 recommended by Python documentation, https://devguide.python.org/documenting/#sections
• (if we expand to a longer list) 13 recommended in https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html#sections