Add extra requirements.txt ref to the Docs

[armin-x86]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Two scripts within the project, inventory.py and download_hash.py, rely on YAML parsing functionalities. Without the ruamel.yaml library referenced in requirements.txt, users on freshly installed systems or Python environments will encounter a ModuleNotFoundError when trying to execute the command:

CONFIG_FILE=inventory/mycluster/hosts.yaml python3 contrib/inventory_builder/inventory.py ${IPS[@]}

This error occurs because the ruamel.yaml library is not installed by default.
This should be indicated in Installation docs to provide more clarity.

Which issue(s) this PR fixes:
Fixes https://github.com/kubernetes-sigs/kubespray/issues/4331

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[k8s-ci-robot]

Hi @armin-x86. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: armin-x86
Once this PR has been reviewed and has the lgtm label, please assign liupeng0518 for approval. For more information see the Kubernetes Code Review Process.

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[yankay]

/ok-to-test

[tico88612]

Please look at the Kubespray CI - pre-commit job log and fix it.