[Elastic Agent] Document support/no support for EA on AWS Fargate

[lucabelluccini]

We're receiving inquiries about running Elastic Agent on AWS Fargate on readonly root filesystems.

We should document if we support those environment and what are our recommendations as we have a KB article https://support.elastic.co/knowledge/dfbcac88 which shows its technically possible.

[nimarezainia]

thanks @lucabelluccini do you think this needs to be documented in our public docs or just the KB? I'd be tempted to modify the docs to say that Agent needs write-able filesystem (etc) but refrain from calling out a particular technology as not being supported (eg AWS Fargate)