Fix APISIX monitoring guide configuration and metrics

[Yilialinn]

Fix stale APISIX monitoring documentation

• Add a unit test to verify that the fix works. (Not applicable to this documentation-only correction; the Collector configuration, metric-name set, patch, links, and data path were validated directly.)
• Explain briefly why the bug exists and how to fix it.

The APISIX backend monitoring guide still lists metric names that no longer match the current APISIX MAL rules or the Horizon UI APISIX Dashboard. It also omits the APISIX metrics path from the Collector example and does not explain that job_name: apisix-monitoring is part of the current MAL filter contract.

This documentation-only change:

• updates the existing supported-metrics table with the current service, instance, and endpoint metric names;
• adds /apisix/prometheus/metrics to the Prometheus receiver example;
• explains the job_name dependency and retains the E2E-tested skywalking_service static-label pattern;
• corrects setup wording and links detailed Dashboard semantics to Horizon UI; and
• updates docs/en/changes/changes.md.

The change does not modify MAL rules, E2E behavior, Horizon UI templates, or APISIX code.

Validation:

• compared all 23 unique final metric names with otel-rules/apisix.yaml;

• validated the complete Collector configuration with the official otelcol 0.102.1 binary;

• reproduced the APISIX → Collector → OAP → BanyanDB path and the official service, instance, and endpoint metric queries; and

• checked the candidate patch and all Markdown links against the current SkyWalking master branch.

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. (Not applicable; this small documentation correction does not close an existing issue.)

• Update the CHANGES log.

[wu-sheng]

You should put your changes in the Doc section, not here. And should be in the last.