Refactor Pulp repo definitions and add more Pulp documentation

[wtripp180901]

• Host to deploy Pulp on now defined by pulp_server group rather than extravars for the deploy-pulp.yml adhoc
• Pulp now managed by a systemd service
• Group to trigger Pulp sync during fatimage has been renamed from pulp to pulp_site
• Format of dnf_repo_timestamps.yml has been changed, with entries now containing all information required for both dnf_repos and pulp_site roles
• dnf_repos file/package definitions and pulp repo path definitions now templated from structure in dnf_repo_timestamps.yml (no longer need to be defined manually in roles)

Breaking Change
The new Pulp repository definitions may conflict with repos already synced on your Pulp server. The easiest way to remediate this is to delete your existing Pulp state and redeploying. This can be done by

1. Stopping the Pulp Podman container on the Pulp host
2. Deleting the state directory at /home/rocky/pulp
3. Re-run the ansible/adhoc/deploy-pulp.yml playbook

[wtripp180901]

https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17044219288

[wtripp180901]

Got gpg check errors from build
Failed to validate GPG signature for epel-release-8-18.el8.noarch: Public key for epel-release-8-18.el8.noarch.rpm is not installed
so we do need the checks disabled for now, but maybe we need a follow-on PR to download the upstream keys

[wtripp180901]

https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17065933041

[wtripp180901]

https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17067130289

[wtripp180901]

https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/17069041388

[sjpb]

Please see comment/linked PR for ditching the top-level keys in the repo datastructure.

[wtripp180901]

Added the suggested changes from here #557 while I was updating the docs

[sjpb]

LGTM, I just have grammar change and suggestion re. timestamps.yml naming

[sjpb]

LGTM