Use a docs branch for auto generated CLI docs

[willemneal]

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There is no reason that the docs should block merging PRs. It slows down development wastes CI cycles. Instead it should be a CI action that runs on merges to main. This is very standard for projects with docs generated and github already supports it.

Originally posted by @willemneal in https://github.com/stellar/stellar-cli/pull/1418#pullrequestreview-2148644938