Fix rendered towncrier changelog to be consistent with the way towncrier does it

[astrofrog]

When running towncrier, the top_line is output before the rendered content based on the template - this means that {top_line} should not be included in the template otherwise the title appears twice. See:

• remove header that causes towncrier rendering issues astropy/astropy#12083

for a fix to the template in astropy core to avoid this issue. See also this code in towncrier which shows the top line is output before the rendered content.

This PR then updates sphinx-changelog to also output the top line separately from the rendered content. With this and astropy/astropy#12083, the astropy docs finally build with no errors related to section headings for me.

This is a known issue in towncrier:

• title repeated twice when title_format is set twisted/towncrier#346