docs: some small changes

[zimbatm]

Another pass on random things I saw could be improved in the docs.

• docs: document the global format
• docs: focus on the default experience in the quickstart
• docs: link to treefmt-nix instead of duplicating the examples
• docs: put motivation first

[brianmcgee]

@mergify queue

[mergify[bot]]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at *3f44ec45421f2324cb120f7978938907284f55dc*