docs: restructure observability navigation and overview (#49505)

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## Stack

Draft stack extracted from `docs/monitoring`. Merge bottom-up. The
troubleshooting *catalog* rewrite (`content/troubleshooting` and the
Diagnosing UI) stays out of scope.

1. #49503 move inspect and advisors
2. #49501 split Studio logs from ClickHouse queries
3. #49500 treat reports as signal dashboards
4. #49502 add Observe the data hub
5. #49506 add agent setup components
6. #49504 add hire-an-agent templates
7. **#49505** restructure observability nav, overview, Detecting, and
flatten Observe the data ← **this PR**

## I have read the
[CONTRIBUTING.md](https://github.com/supabase/supabase/blob/master/CONTRIBUTING.md)
file.

YES

## What kind of change does this PR introduce?

Docs update. Top layer in the observability stack.

## What is the current behavior?

The section is still titled Monitoring and Debugging, with a Debugging /
Monitoring split that does not match the new pages. The debugging guide
is still the master layer-isolation + symptom table. Observe the data is
split into “what data” vs “where to observe it,” which duplicates the
source pages.

## What is the new behavior?

- Section title is Observability
- Overview groups Observe the data, Detect and resolve, Hire an agent,
and Export
- **Observe the data is flattened by source.** Logs, Metrics API,
Database, Advisors, and Reports each list where to read that source.
There is no separate MCP/API/CLI/Studio nav group.
- **Observe vs Detecting:** Observe is the catalog (what exists, how to
access it). Detecting is how to *use* those sources to pick up a Health
/ Security / Performance / Usage signal. Named errors skip to
Diagnosing.
- Studio Logs sits under Logs. Reports sits beside the other sources.
- Troubleshooting stays in the global menu and also appears as
Diagnosing under Detect and resolve

## Additional context

This is the last PR in the stack. Together the seven PRs reconstruct the
`docs/monitoring` observability IA and guide content, without shipping
the troubleshooting catalog overhaul.
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---------

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Saxon Fletcher <SaxonF@users.noreply.github.com>
Co-authored-by: Nik Richers <nik@validmind.ai>

apps/docs/scripts/search/sources/index.ts

@@ -140,7 +140,7 @@ export async function fetchCliLibReferenceSource() {
 export async function fetchLintWarningsGuideSources() {
   return new LintWarningsGuideLoader(
     'guide',
-    '/guides/monitoring-and-debugging/advisors',
+    '/guides/observability/advisors',
     'supabase',
     'splinter',
     'main',