Add a Nox session to build the changelog

[namurphy]

We should add a nox session for using towncrier to build the changelog. At the same time, we should update the changelog guide accordingly to include the command to build the changelog.

We currently have the commands to build the changelog defined in a workflow. The nox session shouldn't include the git commands, unless we include sessions with and without git, or had a session posarg to provide this option.

✨ Creating nox sessions for tasks like this will help make it so that we can do all the GitHub workflow tasks locally as well, which can help with maintaining and debugging the code for these tasks.

I'll assign this to myself to go along with the other nox PRs I'm doing.