ECE AWS load balancers, ipv6 and proxy protocol v2 tutorial

[eedugon]

DO NOT MERGE UNTIL ECE RELEASE IS CLARIFIED

Summary

Adding new doc from https://github.com/elastic/cloud/pull/153120 into docs-content as a tutorial.

Original content by @brendandw refined in this commit (for your review):

• Includes a small clarification between client IP preservation vs propagation with proxy protocol v2.
• Update the existing installation procedure, enforcing one-by-one proxy container recreation and validation to reduce outage risks.
• Cross-links to and from other ECE docs.

Pending:

• Clarify to what ECE version this applies.
• Clarify since when the ECE installation script and proxy supports --proxy-protocol-version 2 and --proxy-protocol-lenient flags, because our docs are already mentioning proxy protocol v2 support in the ece-load-balancers.md doc.
• Determine if we need to update standard installation documents to mention the possibility of needing / wanting proxy protocol version 2, as that should be configured at installation time preferably.

Generative AI disclosure

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

• Yes : cursor with GPT model
• No

[github-actions]

Vale Linting Results

Summary: 1 warning, 1 suggestion found

⚠️ Warnings (1)

File	Line	Rule	Message
deploy-manage/deploy/cloud-enterprise/ece-load-balancers.md	47	Elastic.Spelling	'balancer's' is a possible misspelling.

💡 Suggestions (1)

File	Line	Rule	Message
deploy-manage/deploy/cloud-enterprise/ece-networking-prereq.md	15	Elastic.WordChoice	Consider using 'deactivated, deselected, hidden, turned off, unavailable' instead of 'disabled', unless the term is in the UI.

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[github-actions]

🔍 Preview links for changed docs

• deploy-manage/deploy/cloud-enterprise/ece-ipv6-aws-setup.md
• deploy-manage/deploy/cloud-enterprise/tutorials.md
• deploy-manage/deploy/cloud-enterprise.md
• deploy-manage/deploy/cloud-enterprise/ece-load-balancers.md
• deploy-manage/deploy/cloud-enterprise/ece-networking-prereq.md

[eedugon]

Copied from https://github.com/elastic/cloud/pull/153120#pullrequestreview-3998097378, by @jakommo :

I think this only exists on the first ECE host that was installed and I've also seen that customers removed it for security reasons.
Should we default to set the PW manually with a note that it can be sourced from this file if it exists on the host?

There's no need to provide this step, maybe just as a tip in case the user doesn't know the admin password. Ideally we should link to other official docs if the reader needs to know more about the admin password. For the tutorial this is already reported as a prerequisite.