[DOCS] Endpoint ES connection error troubleshooting

[dasansol92]

Description

Related PR: elastic/kibana#140167

A troubleshooting docs page for when Endpoint has an ES connection error.

Notes

• Add all appropriate labels to the issue, especially the version number label.
• Be sure to add any necessary screenshots, code text or console commands for clarity.
• Include any conditions or caveats that may affect customers.

[dasansol92]

FYI - @kevinlog

[joepeeples]

The error to be documented is dependent on the v2 protocol for Elastic Agent, so we can deprioritize writing the troubleshooting docs until the v2 Agent is released.