Unbury the "how we make changes" page from its current hard-to-find location

[hollsk]

r.e. https://github.com/springernature/frontend-playbook/blob/5b58578a830b6f04975e72d4aba8c5e4eaef5ff0/practices/house-style.md#making-changes-to-the-house-style

in #361 @dotcode quoth:

I think that particular part of the House Style page is almost something you'd expect to see in a Contributing.md. I think this is one of those areas where there's a large degree of overlap between the README/Contributing/house-style pages. That's opening a (small) can of worms, I guess. I think this probably needs looking at, we might be able to figure out a way to reduce the overall number of pages and make navigation easier. Maybe? Worth thinking about, at some point. What do you think?

(i agree with him)