[fortinet_fortigate] Update Fortigate documentation

[mjwolf]

Proposed commit message

Update documentation for the fortinet_fortigate integration. The documentation now more complete, with expanded information on supported use-cases and data types that can be collected, more details on how to completely configure the integration, and more troubleshooting information.

This also includes a new packages/fortinet_fortigate/docs/knowledge_base/service_info.md file, which will be included in the built package, and can be used by other tools which use information from the package, such as LLM tools.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
  - [ ] I have verified that any added dashboard complies with Kibana's Dashboard good practices

Screenshots

Screenshot of the updated documentation as rendered in Kibana:

[elastic-vault-github-plugin-prod]

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[elasticmachine]

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