Add vision and design principles doc

[jesseduffield]

I want to spell out the design principles behind Lazygit so that our priorities are clear and it's easier to make UX decisions (especially for early contributors).

It's not possible to codify this in a way that will resolve all disputes but I think it's worthwhile to enumerate some of the things that keep coming up in our discussions about X or Y feature.

This is a very rough draft (effectively a brain-dump). So I expect plenty of feedback :)

CC @stefanhaller @karimkhaleel @mark2185 @afhoffman @AzraelSec

• Please check if the PR fulfills these requirements

• Docs have been updated if necessary

[codacy-production]

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Coverage variation	Diff coverage
Report missing for b0a766c1	✅ ∅

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	Coverable lines	Covered lines	Coverage
Common ancestor commit (b0a766c)	Report Missing	Report Missing	Report Missing
Head commit (77c1b93)	50681	43629	86.09%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4053)	0	0	∅ (not applicable)

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[karimkhaleel]

Suggested change

	- Comformity with git
	- Conformity with git

[karimkhaleel]

Document how to solve various git problems with Lazygit

Strongly agree with this point. Too often I see people struggling with their repos because they are not even aware of many of the ways you can manipulate commits and branches. I really like how Lazygit allows Git's features to be very easily accessible and not behind a completely opaque UI (i.e you can see exactly what git commands are running in the background).