[docs] Migrate docs from AsciiDoc to Markdown

[colleenmcginnis]

Migrate docs from AsciiDoc to Markdown. The preview can be built after #2634 is merged.

[JoshMock]

Caught a couple typos. I'm excited to see the preview before making any bigger suggestions!

[marciw]

preview: https://docs-v3-preview.elastic.dev/elastic/elasticsearch-js/pull/2635/reference

[marciw]

@JoshMock even though we have some build errors, this is probably ready to go. The preview is here https://docs-v3-preview.elastic.dev/elastic/elasticsearch-js/pull/2635/reference if you want to take a look! (no rush)

... and this bottom part of the nav is a known issue that will be fixed when we build all the docs together instead of just one piece:

[marciw]

(to clarify, docs eng says the build errors are fine to overlook, as long as the preview looks ok to you.) 🤘

@JoshMock even though we have some build errors, this is probably ready to go. The preview is here https://docs-v3-preview.elastic.dev/elastic/elasticsearch-js/pull/2635/reference if you want to take a look! (no rush)

... and this bottom part of the nav is a known issue that will be fixed when we build all the docs together instead of just one piece:

[JoshMock]

I spot-checked several pages and everything looks great! 👏