Use correct header level when splitting rules list into sub-lists with --split-by

[bmish]

Header levels should increment by one for nested sections. When splitting the rules list/table into sub-lists, we should use a header level that is one greater than the previous header level seen in the file.

Example:

## Rules

<!-- begin auto-generated rules list -->

### some sub list 1

...

### some sub list 2

...

<!-- end auto-generated rules list -->

Before this fix, we always assumed the sub-lists should use ### since the ## Rules section usually uses a level 2 header. Now, we adjust the sub-list header level based on the surrounding context.