Add ML modules to Nginx readme

[lcawl]

What does this PR do?

Adds a table to the readme for the Nginx module, which lists the machine learning modules that are included in the integration. The names and descriptions for these jobs are found in https://github.com/elastic/integrations/blob/main/packages/nginx/kibana/ml_module/nginx-Logs-ml.json

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.

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates Add ML modules to Apache integration readme #2640

Screenshots

[elasticmachine]

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• Start Time: 2022-02-09T17:17:40.175+0000

• Duration: 20 min 8 sec

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🤖 GitHub comments

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[peteharverson]

LGTM

[bmorelli25]

LGTM