main/fish: make util-linux a dep of fish-doc and add a README

It is required to have util-linux to use the `help` command in any of
the builtins fish provides as the internal `__fish_print_help`  function
uses the `ul` program that `util-linux` provides.

`__fish_print_help` also returns early if there are no manpages
installed which are present in `fish-doc` so can we can kludingly deal
with this by tying the `util-linux` dependency to the -doc subpackage.

This is not perfect as many users that want `fish-doc` don't want
`util-linux` but it is better than giving a free `util-linux` dependency
to anyone that installs `fish`
7 jobs for !16787 with fish in 1 minute and 45 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Lint
passed #292504
docker-alpine x86_64
lint

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  Build
passed #292509
docker-alpine ci-build aarch64
build-aarch64

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passed #292510
docker-alpine ci-build armv7
build-armv7

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docker-alpine ci-build ppc64le
build-ppc64le

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docker-alpine ci-build s390x
build-s390x

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docker-alpine ci-build x86
build-x86

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passed #292505
docker-alpine ci-build x86_64
build-x86_64

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