Fix SecureGuard installation docs

[abuango]

Fixes found while installing SecureGuard from these docs end to end.

installation/

• Add a Prerequisites notice stating the registry is private and access is by invitation, with a route to credentials via sales. Removes the duplicate statement from the pull-secret bullet below it.
• Add kubectl create namespace before the secret command — following the page top to bottom failed on its first command.
• State the node minimum (3 × 2 vCPU / 4 GiB). The chart's post-install NOTES say it; no doc page did, which is too late to pick a cluster.
• Move What the Chart Deploys above Deployment Modes, so you know what you are installing before you install it.
• Add an RBAC binding step to Install Order; the chart ships no bindings, so a user who logs in without one gets 403 on everything.
• Add helm test secureguard as the verification step. The chart ships the test and the NOTES advertise it; it appeared on no doc page.
• Document --version pinning on the Managed install.
• Add an Uninstalling section: the OpenBao PVCs, <release>-openbao-keys, <release>-dex-admin, and the five CRDs installed by a pre-install hook all outlive helm uninstall.
• Note that the unseal-key backup writes the key shares to disk in plain text.

getting-started/

• Add creating the pull secret as step 1, with the namespace; drops --create-namespace from the install, and renumbers the rest.
• Add a credentials notice to Prerequisites.

troubleshooting/

• Add Pods Stuck in ImagePullBackOff as the first General Issues entry. It is the most likely first failure and was undocumented.

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