feat: Allow user customization of release notes template

[bryant-finney]

Description

Allow end users to override the default Jinja2 template for rendering release notes.

If a file named .release_notes.md.j2 exists in the configured template_dir, it is used to render the release notes of a given VCS release.

Implements #660

Checklist

• test implementation: see the following commits for this feature's two acceptance tests:
  1. c3f54c8
  2. 476cedb
  3. d648d79
  4. 3565dcd
• feature implementation: see 63d952a for the implementation of this feature
• documentation updates:
  • 8dc1283 and 884bbf8 update the changelog_templates.html Sphinx page to describe how to use this feature
  • 3f2a96a updates how the --noop argument is handled in the version and changelog commands to display the rendered release notes

Misc. Notes

• f41fc57 was needed to fix a couple of preexisting pre-commit failures
• 1484305 was later reverted by 63d952a
• 19a01f0 and baf050a cleaned up some of the type annotations and pathlib usage
• in addition to the acceptance tests, I also validated this change by running the following in one of my projects:

  # install python-semantic-release from my local clone of the project
  pip uninstall -y python-semantic-release    # remove the distribution from PyPI
  pip install -e ../python-semantic-release
  
  # create a release notes template
  cat <<EOF >templates/.release_notes.md.j2
  ## What's Changed
  {% for type_, commits in release["elements"] | dictsort %}
  {%- if type_ != "unknown" %}
  ### {{ type_ | capitalize }}
  {% for commit in commits %}
  * {{ commit.descriptions[0] }} by {{commit.commit.author.name}} in [`{{ commit.short_hash }}`]({{ commit.hexsha | commit_hash_url }})
  {%- endfor %}
  {% endif %}{% endfor %}
  EOF
  
  # simulate creating a new release
  semantic-release --noop version

[bryant-finney]

@danth @relekang @bernardcooke53 could I ask for some help with merging this PR?

Based on a parallel PR in my fork (bryant-finney#1), I think it's ready to merge now

CC @jmgate

[jmgate]

Excited to be able to use this feature when it makes it in 😀

[bernardcooke53]

This looks really good - I spent a fair bit of time going over all the changes and testing this locally, I think it's really easy to follow, well-commented and well-tested, thanks for all the effort! 🚀
I've left a couple of comments/questions but nothing major