Update instances config template

[alopezz]

What does this PR do?

• Update the instances template. Fix some errors and improve the doc.
• Update the example config for integrations accordingly.

Motivation

AI-2648

Additional Notes

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[codecov[bot]]

Codecov Report

Merging #12950 (2c03c9c) into master (cd4cd84) will increase coverage by 0.08%. The diff coverage is n/a.

Flag	Coverage Δ
datadog_checks_dev	80.13% <ø> (+1.40%)	:arrow_up:

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