docs: add upstream provider status example

[koko3tallah]

What changed

• document a copy-paste Gatus endpoint for monitoring a cloud or SaaS provider through OutageDeck’s normalized official-status API
• use a conservative 10-minute interval, bounded client timeout, and native JSON body condition
• explain all non-operational states and state explicitly that official-feed evidence complements rather than replaces direct application health checks
• add the new section to the README table of contents

Why

Gatus already supports JSON-path body conditions, but users monitoring their own applications do not currently have an example for correlating a failure with an upstream provider incident. This recipe lets them reuse their existing Gatus dashboard and alerting configuration without a new parser, API key, or local agent.

Validation

• git diff --check
• go build -o /tmp/gatus-outagedeck-qa .
• started the built Gatus binary with the exact documented endpoint configuration
• verified /api/v1/endpoints/statuses returned HTTP 200 and both documented conditions succeeded:
  • [STATUS] == 200
  • [BODY].data.currentStatus.code == operational

The broader go test ./config/... run reached the repository’s live-network integration checks, where twin.sh and example.org were unreachable from this environment. The README-only change does not touch those tests or application code.

Disclosure: I am affiliated with OutageDeck.