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

[adiroiban]

Just a small comment... and mauybe this should be a .removal type of fragment.

It should explain the existing users should manually update existing NEWS file to match the new H2 format.

Suggested change

	Markdown template: Render category headers as h2 when no sections are present.
	When no sections are present, the default Markdown template now renders the category headers as H2. In previous versions it was rendered as H3.

[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