Allow overriding of default config values.

Is your feature request related to a problem? Please describe.

I'm currently working on a custom theme and want to override a config default value. When calling app.add_config_value in my setup function, i get ExtensionError from Config.add.

Describe the solution you'd like

It would be nice if Sphinx.add_config_value respective Config.add accepts a flag whether to override the value if it already exists.

Describe alternatives you've considered

As workaround i update the value on Config.values directly, but i consider it a hack.

def setup(app):
    ...
    hpi = app.config.values['html_permalinks_icon']
    app.config.values['html_permalinks_icon'] = ('#', hpi[1], hpi[2])

Start by tracing Sphinx.add_config_value and Config.add, focusing on how existing configuration names produce ExtensionError. Review the Config.values workaround in the issue and the surrounding configuration behavior. Done means an explicit override option is supported without breaking existing defaults or duplicate-name handling.