fixed #6577 issue

[javatcoding1]

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Pull Request Description
This pull request addresses issue #6577, which involved incorrect Markdown implementation for heading levels. The following changes were made:

Updated the autoformatRules in volto/packages/volto-slate/src/editor/plugins/Markdown/constants.js to align with CommonMark specifications.
Modified the autoformat rule for a single # to generate an

tag, ## to generate

, and so on, ensuring consistency with Markdown standards.
Verified that these changes maintain compatibility with existing functionality and do not introduce regressions.

Closes #6577.

[mister-roboto]

javatcoding1 the email address in your commit does not match an email in your GitHub account. Thus it is impossible to determine whether you have signed the Plone Contributor Agreement, which is required to merge this pull request.

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