Plone-sphinx-theme

Internal changes

- Unpin mermaid and specify which version of the JavaScript library to load. stevepiercy [Issue 23](https://github.com/plone/plone-sphinx-theme/issues/23)

- Clean up release. stevepiercy

New features

- Add support for Mermaid Sphinx extension. stevepiercy [Issue 22](https://github.com/plone/plone-sphinx-theme/issues/22)

Internal changes

- Change build-system to use setuptools. Fix manifest. stevepiercy [Issue 21](https://github.com/plone/plone-sphinx-theme/issues/21)

Internal changes

- Steal towncrier config from Volto. Clean up `CHANGES.md`. stevepiercy [Issue 18](https://github.com/plone/plone-sphinx-theme/issues/18)
- Fix towncrier template path. stevepiercy [Issue 19](https://github.com/plone/plone-sphinx-theme/issues/19)

Internal changes:

- Add .gitkeep to news directory to preserve it upon releases. stevepiercy ([Issue garden](https://github.com/plone/plone-sphinx-theme/garden))
- Fix CHANGES.md content for automatic updates upon releases. stevepiercy ([Issue garden1](https://github.com/plone/plone-sphinx-theme/garden1))
- Add GitHub action to deploy documentation to Read the Docs. stevepiercy ([Issue garden2](https://github.com/plone/plone-sphinx-theme/garden2))
- Add pull request preview on Read the Docs. stevepiercy ([Issue garden3](https://github.com/plone/plone-sphinx-theme/garden3))
- Clean up conf.py. stevepiercy ([Issue garden4](https://github.com/plone/plone-sphinx-theme/garden4))

Documentation:

- Fix glossary syntax. stevepiercy ([Issue glossary](https://github.com/plone/plone-sphinx-theme/glossary))