docs: Add a Jujutsu from first principles doc.

[PhilipMetzger]

This came up recently in Discord as a explanation was requested without referencing Git, which all our other documents do.

I think this may be useful for the initial page on the website.

Rendered

I'll probably need help from @martinvonz and @ilyagr to expand on this.

[joyously]

Seeing

as there are always multiple people working on the same file(s) at the same time

made me think of AutoMerge and then Grace.