[ENH/FIX] Support headings in nested parse

[minijackson]

This fixes using Markdown headings in Sphinx objects, like the object, describe, py:data directives, etc.; which fixes #1050.

In summary, this should now work:

# Title

```{object} foo
bar
# Title in object
wunderbar
## Sub-title in object
wunderbar
```

This works by adding the heading to the temp root parent instead of at the end of the previous section from the main document.

I've also added a change to the "heading to rubric" conversion in nested headings, whose levels now will be offset by the current heading level. So this:

# Title
## Sub-title
> Quote:
> # Title in quote
> Content.

will generate an h3 rubric inside the blockquote (offset by the "Sub-title" heading).
This makes it consistent with the way it works with Markdown headings inside objects (which is consistent with RST's headings inside objects).
Tell me if you think this change is appropriate, if not I can always remove the commit.

I've also used the heading-level option from Sphinx' rubric directive, so that Sphinx generate appropriately leveled headers from those rubrics.

Edit: for the offsetting of nested headers, I had to change a part of the documentation, because an example generated a level 4 headers, whose id wasn't generated due to a myst_heading_anchors value too low.