Converting document from hard whitespace indentation to org-indent-mode

[Max Brieiev]

Several of my documents use indentation to line up with the headings,
like below:

* Top level headline
  paragraph
  + list item lorem ipsum
    dolor sit amet
** Second level
   some text
*** Third level
    more text

I would like to strip leading whitespace, so that I could switch to
=org-indent-mode= to have a nice view of the document instead of using
hard whitespace indentation.

Basically I want to:
- strip leading whitespace,
- but preserve the indentation of lists and src blocks
- refill all paragraphs according to what =org-fill-paragragh= would do.

How would I do it programmatically? My current Elisp knowledge is poor.

Thanks in advance.