rst not recognized as rst when headings skip a level

[timotheecour]

this won't render as rst:

Heading1
========

Heading2
--------

Heading3
~~~~~~~~

Heading4
========

Heading5
~~~~~~~~

this will:

Heading1
========

Heading2
--------

Heading3
~~~~~~~~

Heading4
========

Heading4b
---------

Heading5
~~~~~~~~

this is the root cause for https://github.com/nim-lang/Nim/blob/devel/doc/manual.rst not rendering properly as of https://github.com/nim-lang/Nim/blob/bbaa05dc8245d297cc21dd6b41a0472795fa0830/doc/manual.rst#L1 at least

(found after a painful binary search)

links

• https://docutils.sourceforge.io/docs/ref/rst/restructuredtext.html
• https://github.com/nim-lang/Nim/pull/17092
• https://github.com/nim-lang/Nim/issues/17091

[slgraff]

Another report of this: https://github.zendesk.com/agent/tickets/2163763

[github-actions[bot]]

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