Consider changing the term "National SteerCo" to "Flux SteerCo" to support international movements

[sventhebarbarian]

At the moment it seems "Flux SteerCo" and "National SteerCo" are used interchangeably see:

https://docs.flux.party/en/latest/docs/contributing/index.html

I would suggest in the docs we use one term than encompasses international movements under the umbrella SteerCo, my preference is "Flux SteerCo" as it is already in use, but we could come up with another term.

If this is agreeable can I begin making changes to the docs to reflect this?

[XertroV]

Aus National SteerCo is full general name. Whatever we pick should be compatible with i18n internationalisation so Flux steerco is ambiguous