[docs] Remove Pulsar Manager docs and rename the Dekaf UI page to administration-ui

[lhotari]

Motivation

Apache Pulsar Manager is no longer maintained, and using it is not recommended. The documentation still presents it as the web UI for managing Pulsar, including a dedicated page and a "Use Pulsar Manager to manage the cluster" step in the Helm quick start guides. The Dekaf UI page is the place to point users who want a web-based UI. This PR also renames that page to a tool-neutral administration-ui and mentions an alternative UI.

Modifications

The following changes apply to the next docs (docs/) and to the 5.0.x, 4.2.x, 4.1.x and 4.0.x versioned docs:

• Delete administration-pulsar-manager.md and remove it from the sidebars. The 4.1.x and 4.0.x sidebars did not list the existing administration-dekaf-ui page, so the Pulsar Manager entry is replaced with the Dekaf UI entry there.
• getting-started-helm.md and kubernetes-helm.md: remove the "Use Pulsar Manager to manage the cluster" step and renumber "Use Prometheus and Grafana to monitor cluster" from Step 5 to Step 4. Update the step overview tip to match. Remove the Pulsar Manager pod and service from the sample kubectl output, the pulsar_manager values tip and the Pulsar Manager login mention.
• helm-deploy.md: remove pulsar_manager from the components and images examples, the "Access Pulsar cluster" list and the NodePort example. The Pulsar Helm chart already disables pulsar_manager by default.
• deploy-ibm.md: remove the Pulsar Manager login mention.
• deploy-monitoring.md: remove the "Pulsar per-topic dashboard" subsection, which only linked to the Pulsar Manager page.

Redirects: static/.htaccess has a new rule:

RewriteRule "^docs/(.+/)?administration-pulsar-manager/?$" "/docs/administration-ui/" [R=301,DPI,L]

The rule runs after the existing "skip if file exists" check. This means /docs/administration-pulsar-manager/, /docs/next/administration-pulsar-manager/, /docs/5.0.x/…, /docs/4.2.x/…, /docs/4.1.x/… and /docs/4.0.x/… redirect to the administration UI page. Older versioned docs that still include the page (3.x and earlier) continue to serve it. The existing administration-dashboard redirect chains through the new rule for the affected versions.

Rename the Dekaf UI page to administration-ui

In every version that has the page (next, 5.0.x, 4.2.x, 4.1.x, 4.0.x and 3.0.x):

• Rename administration-dekaf-ui.md to administration-ui.md, with the title "Administration UI", and update the sidebars and internal links.
• Organize the page by tool: a "Dekaf UI" section (h2) with its "Features" (h3), and an "Alternative" section (h2) listing DataStax Pulsar Admin Console, which supports OpenID Connect authentication. Both projects are licensed under the Apache License 2.0, which the page now states.
• Redirect the old URLs to the renamed page in the same version:

RewriteRule "^docs/(.+/)?administration-dekaf-ui/?$" "/docs/$1administration-ui/" [R=301,DPI,L]

Out of scope for this PR (unchanged): older versioned docs (3.3.x and earlier), the blog, the release notes (release-notes/pulsar-manager.md), the download page's Pulsar Manager section, and the ecosystem and resources entries under data/.

✅ Contribution Checklist

• I read the contribution guide
• I updated the versioned docs