First beta: update generation of next "What's New"

[hugovk]

For the first beta like 3.14.0b1, we create a new 3.14 branch and main is the for 3.15.

The release automation creates the new "What's New in Python 3.15" Doc/whatsnew/3.15.rst file from a template.

This PR does two things.

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First, add a missing newline to the template to fix:

Doc/whatsnew/3.15.rst
  142: Bullet list ends without a blank line; unexpected unindent. [docutils]

https://github.com/python/cpython/actions/runs/14888234804/job/41813532717?pr=133588

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Second, right now, we create the new file Doc/whatsnew/3.15.rst but don't add it to the toctree in Doc/whatsnew/index.rst, and get an error:

/home/runner/work/cpython/cpython/Doc/whatsnew/3.15.rst: WARNING: document isn't included in any toctree [toc.not_included]

So this PR also updates Doc/whatsnew/index.rst with the new 3.15.rst entry.