Fix section depth in markdown template

[flying-sheep]

Description

Fixes #663

Checklist

• Make sure changes are covered by existing or new tests.
• For at least one Python version, make sure test pass on your local environment.
• Create a file in src/towncrier/newsfragments/. Briefly describe your
  changes, with information useful to end users. Your change will be included in the public release notes.
• Make sure all GitHub Actions checks are green (they are automatically checking all of the above).
• Ensure docs/tutorial.rst is still up-to-date.

[adiroiban]

Thanks for the PR.

Changes looks good... but this might be considered a backward incompatible change.

Users that already have NEWS files, will have them generated with H3 for previous releases.

I think that this might be ok, but we should communicate this change to end users.

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I was expected to also see some updates to the test docstrings, to explain that the H2 render is important.

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I hope that someone who cares about MD support in towncrier will have some time and do a review of this PR

[flying-sheep]

Hi @adiroiban, I commented a bit on the bigger problem here: #663

maybe you’d like to chime in there before I invest more time into this fix

[adiroiban]

This looks good.

I think that trunk needs to be updated to fix the read the docs build.

[flying-sheep]

Huh, never saw that one before!

ERROR: THESE PACKAGES DO NOT MATCH THE HASHES FROM THE REQUIREMENTS FILE. If you have updated the package versions, please update the hashes. Otherwise, examine the package contents carefully; someone may have tampered with them.
    pip from https://files.pythonhosted.org/packages/c9/bc/b7db44f5f39f9d0494071bddae6880eb645970366d0a200022a1a93d57f5/pip-25.0.1-py3-none-any.whl:
        Expected sha256 c46efd13b6aa8279f33f2864459c8ce587ea6a1a59ee20de055868d8f7688f7f
             Got        2f15a3708f631f768bfdc86dbcbfc1b5a31ce7097c5858b011f36a909bb3d4de

[adiroiban]

Sorry for the delay... I was quite busy lately.

The code from here is ok... it might be just a Read The Docs issue... I triggered a re-run

Many thanks for your help!