[FEATURE] Add documentation

[zhongnansu]

I see the work here #104 removes the doc folder. I wonder is there a plan to add it back? My recent work that needs me to look into this library in detail, but since there's no doc. I am still referring to the elastic-js client at 7.10. Can you confirm that 7.10 doc still makes sense, except the naming?
https://github.com/elastic/elasticsearch-js/blob/7.10/docs/observability.asciidoc

[zhongnansu]

Oh I remember I asked this question previously. It's 7.14 that we forked from, right?

[kavilla]

Hey @zhongnansu, it was forked from 7.14 but has changed. I would be hesitate to set a precedent that if the documentation doesn't exist here then check the legacy repo. We should consider opening a bug in the documentation repo so that we may add it to this doc: https://opensearch.org/docs/latest/clients/javascript/

[PatrykMilewski]

Bump, it would make use of this lib way easier, instead of jumping back to the web docs

[dblock]

@PatrykMilewski care to contribute?