Updating and consolidating our build guides

[Boudewijn Rempt]

I've made https://invent.kde.org/documentation/docs-krita-org/-/merge_requests/124 -- I've made one page with build instructions for all platforms, the simplest instructions I could create. It needs checking though, since I might be missing something, somewhere. The guides for building krita with docker and with pre-built windows deps are moved, and linked to at the end of the document.
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https://www.krita.org