Respect user configuration and don't overwrite

[a-zw]

When reusing docs-as-code for internal/proprietary purposes, it is rather annoying that the score sphinx extensions overwrite configuration variables explicitly configured in conf.py. Instead, they should "play nice" and respect user configuration.

Setting the html_theme is one example. See #414

[MaximilianSoerenPollak]

This is mainly also because conf.py gets loaded first, and then our extensions.

I think some of these can easily be solved by having the extension settings make it so they 'extend' the current conig instead of setting it.
though not sure about all of them.

[MaximilianSoerenPollak]

I guess this issue eclipse-score/reference_integration#145 is also then solved if we implement this?