Add explicit link targets to releases in changelog

As a result of https://github.com/pypa/twine/issues/683#issuecomment-750902976, I thought one way to make the release tags more useful would be to have a link to the changelog, e.g.



```

git tag -m "https://twine.readthedocs.io/en/latest/changelog.html#3.3.0" 3.3.0

```



However, the [default towncrier template](https://raw.githubusercontent.com/twisted/towncrier/master/src/towncrier/templates/default.rst) doesn't generate [explicit targets](https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#cross-referencing-arbitrary-locations), so right now the link to 3.3.0 is https://twine.readthedocs.io/en/latest/changelog.html#id1. Similar, the features for 3.3.0 is referenced by `#features`, but for 3.2.0 it's `#id14`.



Maybe this could/should be proposed as a change to towncrier, but thought I'd see if there was any appetite for it here.

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