Add standalone agent config examples (System, AWS S3, other basic configs)

[kilfoyle]

As suggested in #65, we need some example configurations for standalone Elastic Agent. This adds to the ones we've already created for Nginx and Apache, via #588 and #584 respectively.

See the following preview pages for the new example configs:

• ingest from a file
• ingest metrics from an HTTP endpoint
• configure multiple outputs for each input
• AWS S3 access logs; storage and request metrics
• host system logs and metrics

NOTE:

• The settings descriptions in the above match the Apache and Nginx examples.
• This also fixes the "API key" docs link in the existing examples.

Closes: #690

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Edit: I had removed the "configure multiple outputs for each input" example, but it's now fixed up and added back in.

[github-actions]

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[cmacknz]

I read through these and they look reasonable to me. Were the configurations tested? I am holding off on approval until I can sanity check a couple of them by actually running them.

[kilfoyle]

Thanks very much @cmacknz! I know these configuration examples are difficult to review. I haven't sanity tested them as I'm not always sure how, but I'm happy to try.

With @nimarezainia, I gathered the configurations by creating a policy in Fleet, adding the relevant integration, downloading the standalone config file, and then identifying the sections we thought important to include in the docs (mainly the inputs and outputs). These are meant to give users a starting point to build up an agent configuration from scratch. If there's anything I can do to help validate them let me know.

[kilfoyle]

@cmacknz Just bumping this PR since now that Serverless is ungated I think it'd be good to have these examples merged into the docs. I'd need some guidance, but if I can help somehow with sanity checking please let me know.

[nimarezainia]

lgtm. thx David

[shainaraskas]

Does this still need a review, or should it be closed?

[kilfoyle]

@shainaraskas, thanks, but no need for you to review this one, and it can stay open.