Add monitoring tutorials to the Operations & Scale sidebar - #3099

What

The Operations & Scale sidebar (/documentation/tutorials-operations/) now lists the three monitoring tutorials that already appear in the main tutorial table:

• Managed Cloud Prometheus Monitoring
• Monitoring with Grafana and Prometheus
• Monitoring with Datadog

Why

The sidebar is built from the section's own pages, and these three tutorials live in other sections (production-operations/ and ops-monitoring/). They are only linked from the table, so they never showed up in the sidebar. Fixes #2954.

How

• sidebar-menu.html already supports type: reference stubs for top-level entries. The section submenu didn't, so I added the same branch there: it links to the referenced page and respects the active state.
• Three stub pages in tutorials-operations/ (ref-*.md, weights 50-52, between "Secure a Self-Hosted Qdrant Instance" and "Kafka Streaming into Qdrant") point at the real pages. They set build.render: never and sitemapExclude, so no stub page is rendered or indexed. They also set learning_kind: reference to override the section cascade (examples), which otherwise fails the build in examples/pages.html.
• The real pages are not moved, so their URLs and aliases stay intact.

How was it tested

Built the site locally with Hugo (full build ~9s) and inspected the generated HTML.

• Before: the /documentation/tutorials-operations/ sidebar had 13 tutorials and none of the three monitoring links.
• After: the sidebar lists all three, in that position, on /tutorials-operations/ and on /tutorials-operations/create-snapshot/, linking to /documentation/production-operations/managed-cloud-prometheus/, /documentation/ops-monitoring/hybrid-cloud-prometheus/ and /documentation/ops-monitoring/hybrid-cloud-datadog/.
• No ref-* pages are rendered into the output, and no ref- strings leak into the html, txt or xml.
• All three link targets build. The old URL /documentation/ops-monitoring/managed-cloud-prometheus/ still exists as an alias redirect, so the table link keeps working.

Not run: a visual check in a browser. No formatter or tests are configured for these files. To build locally I needed two environment workarounds for newer Hugo and sass (HUGO_SECURITY_ALLOWCONTENT and a temporary edit to search.scss). Neither is part of this PR.

Notes

• The sidebar title for the managed-cloud tutorial is that page's own title, "Managed Cloud Prometheus Monitoring". The table calls it "Qdrant Cloud Prometheus Monitoring". Renaming would touch an unrelated page, so I left it.
• Clicking one of these entries opens a page in another section, so the sidebar then switches to that section's menu, as top-level reference entries already do.
• Open PR refactor: cloud docs hygiene ahead of the overhaul #2979 (cloud docs hygiene) only touches cloud-*.md pages, so I found no overlap.

Authored by Mihir Inamdar.