Add more plugins to reduce config downstream

[nabobalis]

Describe the feature

We currently have to add a lot of repeat extensions and config per package:

Examples:

notfound.extension
hoverxref.extension
sphinx_copybutton
sphinx_design
sphinxext.opengraph
sphinx_changelog

Proposed solution

Add these as dependencies to the theme and configure them once here so we do not have to repeat it downstre.

[Cadair]

I am unconvinced that hoverxref is something we want to do globally, but maybe we can just exclude the website and have it everywhere else?

Also I think sphinx-changelog kinda has to be specific to the individual projects? Not all of them use towncrier?

[nabobalis]

I am unconvinced that hoverxref is something we want to do globally, but maybe we can just exclude the website and have it everywhere else?

Makes sense

Also I think sphinx-changelog kinda has to be specific to the individual projects? Not all of them use towncrier?

Do not want all our packages to use the same changelog system?

[Cadair]

Do not want all our packages to use the same changelog system?

maybe 🤷

Either way, I slightly lean towards putting extensions and stuff in a shared config which don't require package specific use or config to work.