DM-30291: Initial configuration of towncrier

[timj]

With this PR (at this time) you get example output of:

Butler v23 2021-05-24
=====================

New Features
------------

- Add a new ``butler retrieve-artifacts`` command to copy file artifacts from a Butler datastore. (`DM-27241 <https://jira.lsstcorp.org/browse/DM-27241>`_)


Performance Enhancement
-----------------------

- Make collection and dataset pruning significantly more efficient. (`DM-30140 <https://jira.lsstcorp.org/browse/DM-30140>`_)


Other Changes and Additions
---------------------------

- `DM-28678 <https://jira.lsstcorp.org/browse/DM-28678>`_


Miscellaneous Changes of Minor Interest
---------------------------------------

- `DM-30291 <https://jira.lsstcorp.org/browse/DM-30291>`_


Butler Datastores
-----------------

Performance Enhancement
~~~~~~~~~~~~~~~~~~~~~~~

- FileDatastore: Improve removing of datasets from the trash by at least a factor of 10. (`DM-29849 <https://jira.lsstcorp.org/browse/DM-29849>`_)


Butler Registry
---------------

New Features
~~~~~~~~~~~~

- Support UUIDs as the primary keys in registry and allow for reproducible UUIDs.

  This change will significantly simplify transferring of data between butler repositories. (`DM-29196 <https://jira.lsstcorp.org/browse/DM-29196>`_)
- Allow registry methods such as ``queryDatasets`` to use a glob-style string when specifying collection or dataset type names. (`DM-30200 <https://jira.lsstcorp.org/browse/DM-30200>`_)

from towncrier --draft --version=23.0.

I think it would be good to define additional types for api changes and performance improvements but so far I've failed to get that working because I think it requires that I redefine all the types (ie it deletes the default types).

[ktlim]

Suggestion: add .github/pull_request_template.md containing at least:

## Checklist

- [ ] ran Jenkins
- [ ] added a release note for user-visible changes to `doc/changes`