install: remove from menu, update in docs (nav and content)

[jorgeorpinel]

Re-do of #381

I think it's confusing to promote the idea of direct installation for CML since it's not the happy path, so I'm proposing to clarifying that in the docs, and to remove the big menu button. Other notes for review below...

See more details starting in https://github.com/iterative/cml.dev/pull/381#pullrequestreview-1185571704 .

[github-actions[bot]]

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