[docgen] runnableExamples causes subsection to be rendered as a section

[nitely]

Maybe this is intended, or the right way is to use .. code-block:: nim with :test:. If that's the case close this.

Example

##[
Section
*********

Subsection 1
###########

]##
runnableExamples:
  doAssert true
##[
Subsection 2
###########

]##

Current Output

"Subsection 2" is rendered as <h1>Subsection 2</h1>

Expected Output

"Subsection 2" is rendered as <h2>Subsection 2</h2>

Additional Information

Using a code block works as expected:

.. code-block:: nim
    :test:
    doAssert true

$ nim -v
Nim Compiler Version 1.3.5 [Linux: amd64]
Compiled at 2020-08-03
Copyright (c) 2006-2020 by Andreas Rumpf

[timotheecour]

reduced:
the root cause isn't runnableExamples, it's the fact that headings are wrong when doc comment is split in multiple parts:

case3 bugs, case3b works

when defined case3:
  ## Basic usage
  ## ===========
  ##
  ## Encoding data
  ## -------------
  ##
  ## Apart from strings you can also encode lists of integers or characters:

  ## Decoding data
  ## -------------
  ##

when defined case3b:
  ## Basic usage
  ## ===========
  ##
  ## Encoding data
  ## -------------
  ##
  ## Apart from strings you can also encode lists of integers or characters:
  ##
  ## Decoding data
  ## -------------
  ##

[timotheecour]

@a-mr it turns out your PR #18256 also fixes this issue; do we need a test case for this before closing? (ideally using toAst)

it's not clear to me whether #18256 actually did add a test for this (at least not ones based on toAst, only html)

[a-mr]

@timotheecour ,
no tests were added in #18256 , so i need to add tests in a follow-up PR.