Add an introduction to FLEET_SERVER_ELASTICSEARCH_CA_TRUSTED_FINGERPRINT

[linghengqian]

• Fixes Add introduction to the FLEET_SERVER_ELASTICSEARCH_CA_TRUSTED_FINGERPRINT environment variable in documentation  elastic-agent#311 .
• Add an introduction to FLEET_SERVER_ELASTICSEARCH_CA_TRUSTED_FINGERPRINT.

[cla-checker-service]

💚 CLA has been signed

[mergify]

This pull request does not have a backport label. Could you fix it @linghengqian? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[kilfoyle]

@linghengqian
Thanks very much for adding this!
I hope you don't mind that I pushed a commit directly to your branch so that the changes will appear in the docs, and I also added very small edits to use variables for the product names. Here's how it appears when I run a test build:

LGTM! 👍

[kilfoyle]

I added a backport-8.9 label so that this will go into the current docs.

[jgaines911]

Is this supposed to be a base64 encode of the sha-256 fingerprint? I would assume not since it does not explicitly say it, but I have seen other elastic documentation calling for the base64 of the ca trusted fingerprint.

[linghengqian]

Is this supposed to be a base64 encode of the sha-256 fingerprint? I would assume not since it does not explicitly say it, but I have seen other elastic documentation calling for the base64 of the ca trusted fingerprint.

• @jgaines911 I remember that this configuration item has existed in the corresponding golang file since Elastic Stack 8.0.x. I just followed its description in other documents to synchronize the instructions here.

• Anyway, maybe you could try submitting another documentation PR?

[jgaines911]

Is this supposed to be a base64 encode of the sha-256 fingerprint? I would assume not since it does not explicitly say it, but I have seen other elastic documentation calling for the base64 of the ca trusted fingerprint.

• @jgaines911 I remember that this configuration item has existed in the corresponding golang file since Elastic Stack 8.0.x. I just followed its description in other documents to synchronize the instructions here.
• Anyway, maybe you could try submitting another documentation PR?

Understood @linghengqian , thank you! I hope to have a call with an elastic engineer next week, so I will definitely ask then.