Enhance Fleet & Agent docs with comprehensive deployment guides

[alexandra5000]

Summary

This PR adds detailed guides for deploying Fleet Server and Elastic Agent, including configuration flags, environment variables, and mutual TLS (mTLS) setup. The new documents provide organized information for various deployment models, ensuring users have access to best practices and prerequisites for successful implementation. Additionally, tips for configuration management and security considerations are included to aid users in effectively managing their deployments.

Closes #4146

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

2. If you answered "Yes" to the previous question, please specify the tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude Sonnet 4.5 via Cursor

[github-actions]

Vale Linting Results

Summary: 12 suggestions found

💡 Suggestions (12)

File	Line	Rule	Message
reference/fleet/deploy-elastic-agent.md	54	Elastic.Acronyms	'SHA' has no definition.
reference/fleet/deploy-elastic-agent.md	113	Elastic.Capitalization	'or other outputs' should use sentence-style capitalization.
reference/fleet/deploy-elastic-agent.md	179	Elastic.Capitalization	'output configuration' should use sentence-style capitalization.
reference/fleet/deploy-elastic-agent.md	236	Elastic.Semicolons	Use semicolons judiciously.
reference/fleet/deploy-elastic-agent.md	239	Elastic.Capitalization	'Mutual TLS (mTLS) configuration' should use sentence-style capitalization.
reference/fleet/deploy-elastic-agent.md	298	Elastic.FutureTense	'will be' might be in future tense. Write in the present tense to describe the state of the product as it is now.
reference/fleet/deploy-fleet-server.md	54	Elastic.Acronyms	'SHA' has no definition.
reference/fleet/deploy-fleet-server.md	151	Elastic.Acronyms	'SAN' has no definition.
reference/fleet/deploy-fleet-server.md	226	Elastic.Capitalization	'Mutual TLS (mTLS) configuration' should use sentence-style capitalization.
reference/fleet/deploy-fleet-server.md	282	Elastic.FutureTense	'will use' might be in future tense. Write in the present tense to describe the state of the product as it is now.
reference/fleet/install-fleet-managed-elastic-agent.md	44	Elastic.FutureTense	'will also' might be in future tense. Write in the present tense to describe the state of the product as it is now.
reference/fleet/install-fleet-managed-elastic-agent.md	46	Elastic.WordChoice	Consider using 'can, might' instead of 'may', unless the term is in the UI.

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/deploy-elastic-agent.md
• reference/fleet/deploy-fleet-server.md
• reference/fleet/add-fleet-server-cloud.md
• reference/fleet/add-fleet-server-kubernetes.md
• reference/fleet/add-fleet-server-mixed.md
• reference/fleet/add-fleet-server-on-prem.md
• reference/fleet/deployment-models.md
• reference/fleet/install-fleet-managed-elastic-agent.md
• reference/fleet/mutual-tls.md
• reference/fleet/secure-connections.md

[alexandra5000]

Hi @karenzone - sorry to call you out like this, but would you have some time to review this?

The idea behind the PR is to create new pages that consolidate the (so far a bit scattered) info on Fleet/Agent deployments. The guides are split into sections by connection type. I decided to keep the platform-specific guides as separate pages and added links to the new pages where I thought it was relevant.

I was mostly working based on the existing docs and configuration examples, so in reality I didn't write a lot of truly "new" content, but still I feel like someone with Agent/Fleet experience should take a look at this - that's why I'm asking you 🙂

[karenzone]

Hi @karenzone - sorry to call you out like this, but would you have some time to review this?

@alexandra5000, I am OOO tomorrow, and returning on Thursday. I can review then.