Document the Elasticsearch output

[ppcano]

Add Elasticsearch to the list of k6 outputs:

• Write the instructions for the Elastic extension similar to other output tutorials.

• List it on the integrations page of the documentation.

• Add to Getting Started - Results Output

[immavalls]

Guide similar to https://k6.io/docs/results-output/real-time/timescaledb/ but for ES based on https://github.com/elastic/xk6-output-elasticsearch. We also have https://github.com/BarthV/xk6-es listed.

[immavalls]

We'll base this on https://github.com/elastic/xk6-output-elasticsearch

It primarily works with ES cloud: https://cloud.elastic.co/. The extension does not support self-signed certificates (we are probably missing the option to configure InsecureSkipVerify, or the ability to pass the RootCA, when using https://github.com/elastic/go-elasticsearch/). ES >=8.0.0 comes with TLS as default. https://www.elastic.co/guide/en/elasticsearch/reference/current/docker.html.

[immavalls]

The first iteration is on https://github.com/grafana/k6-docs/pull/1104. I am initially adding ES output to the docs.

We still need to create a blog showcasing how to run this extension. Once the blog is ready, I'll add a section similar to https://k6.io/docs/results-output/real-time/timescaledb/#read-more.