Add option "--header-offset"

[iptizer]

Is your feature request related to a problem? Please describe.

We are using mkdocs and the generated md should be used as part of our mkdocs documentation.
Thing is, that mkdocs requires a header "first order" for the page name. Everything else has to be 2nd layer or below. Otherwise the index is rendered the wrong way.

Describe the solution you'd like

Add an option --header-offset with default value 0. If set to 1 all headers are offset by one.

Describe alternatives you've considered

Currently we use the following dirtyhack to fix the situation during image build:

RUN sed -i 's/^#### /##### /g' userapi-openapi.md && \ 
    sed -i 's/^### /#### /g' userapi-openapi.md && \
    sed -i 's/^## /### /g' userapi-openapi.md && \
    sed -i 's/^# /## /g' userapi-openapi.md

Additional context

This could also be relevant in case someone wants to add the generated text dynamically in a already present document.