[FR] Add an option to recognize a header of a specific level as a mark for a notebook section.

[XMB-7]

Just use the native heading syntax to represent note sections, without needing to add additional %%section{name}%% markers.

[Quorafind]

How to determine whether a heading in a section is a section separator or not?

[XMB-7]

# heading 1
xxx

## heading 1.1
xxx

## heading 1.2
xxx

# heading 2
...

Only one specific level of headings can be set as the separators, if that is H1, any H2-H6 heading inside H1 section should not also be a separator.