cmd/cue: have a single source of truth for cli docs

[cueckoo]

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Originally opened by @mpvl in https://github.com/cuelang/cue/issues/9

This likely means that the cli docs should be generated from the tool code.

[mvdan]

My 2c: I think we should start moving towards all canonical documentation being on the website. Go already started doing this with https://go.dev/ref/mod, for example. Then go help modules is a short summary with HTTP links, rather than dozens of pages printed out to the terminal, which overwhelm almost anyone.

I also feel like markdown-like styling is helpful. Headers, links, code blocks, and so on. That is technically possible on the terminal with something like man pages, but I think web pages are generally more accessible to the majority of users, particularly when there is so much information.

We could and should keep cue help, but its contents should be limited in size and complexity. Packagers or distros could always ship the HTML pages alongside cmd/cue so that all the docs are available offline.

[jpluscplusm]

Adjacent to this issue, https://alpha.cuelang.org/docs/reference/cli/ is now available.

[mvdan]

I believe we should close this due to the work that has already happened and in favor of automating the structure of https://cuelang.org/docs/reference/cli/ via https://github.com/cue-lang/cue/issues/3204.