Add use_system_theme to documentation

[Justinzobel]

Your use case

What would you like to do?

Please add use_system_theme https://github.com/element-hq/element-web/blob/develop/docs/config.md

Have you considered any alternatives?

No response

Additional context

No response

[t3chguy]

https://github.com/element-hq/element-web/blob/develop/docs/config.md#default-settings covers all settings which can be overridden but use_system_theme is not one of them

[Justinzobel]

I think then we need to alter https://github.com/element-hq/element-web/blob/develop/docs/config.md#customisation--branding to inform admins that to use the Match system theme you must remove the default_theme.