[DOCS] Mention that ESF may result in additional charges

[dedemorton]

What does this PR do?

Adds a statement to the overview page in the ESF docs:

Note that I decided not to put this statement on the index page most users don't see the index page (google sends them to the overview).

Why is it important?

Users need to know in advance if they are doing something that might result in additional charges.

Checklist

• My code follows the style guidelines of this project

Related issues

• Closes [Docs] Review and improve guidance around AWS costs #444

[dedemorton]

@ollyhowell Is this what you had in mind?
@tommyers-elastic Do we need to say more on the troubleshooting page?

[github-actions]

File	Coverage	Lines	Branches		Missing
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main_aws.py	100%	100%	100%	✅
handlers/__init__.py	100%	100%	100%	✅
handlers/aws/__init__.py	100%	100%	100%	✅
handlers/aws/cloudwatch_logs_trigger.py	100%	100%	100%	✅
handlers/aws/exceptions.py	100%	100%	100%	✅
handlers/aws/handler.py	100%	100%	100%	✅
handlers/aws/kinesis_trigger.py	100%	100%	100%	✅
handlers/aws/replay_trigger.py	100%	100%	100%	✅
handlers/aws/s3_sqs_trigger.py	100%	100%	100%	✅
handlers/aws/sqs_trigger.py	100%	100%	100%	✅
handlers/aws/utils.py	100%	100%	100%	✅
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share/json.py	100%	100%	100%	✅
share/logger.py	100%	100%	100%	✅
share/multiline.py	100%	100%	100%	✅
share/secretsmanager.py	100%	100%	100%	✅
share/utils.py	100%	100%	100%	✅
shippers/__init__.py	100%	100%	100%	✅
shippers/composite.py	100%	100%	100%	✅
shippers/es.py	100%	100%	100%	✅
shippers/factory.py	100%	100%	100%	✅
shippers/logstash.py	100%	100%	100%	✅
shippers/shipper.py	100%	100%	100%	✅
storage/__init__.py	100%	100%	100%	✅
storage/decorator.py	100%	100%	100%	✅
storage/factory.py	100%	100%	100%	✅
storage/payload.py	100%	100%	100%	✅
storage/s3.py	100%	100%	100%	✅
storage/storage.py	100%	100%	100%	✅

Minimum allowed coverage is 100%

Generated by 🐒 cobertura-action against 8869243

[kaiyan-sheng]

#449 is merged so this PR should pass the CI now. Thanks!

[ollyhowell]

@ollyhowell Is this what you had in mind? @tommyers-elastic Do we need to say more on the troubleshooting page?

This is great - thanks for picking this up and solving so quickly @dedemorton !