PR #77360 clarifies that a cluster's nodes don't need to be in the same data
center. This adds a similar clarification to the ES introduction docs.
Co-authored-by: David Turner <david.turner@elastic.co>
* Adding documentation for near real-time search.
* Adding link to NRT topic and clarifying some text.
* Adding diagrams and incorporating changes from David T.
* [DOCS] Add introduction to Elasticsearch.
* [DOCS] Incorporated review comments.
* [DOCS] Minor edits to add an abbreviated title and cross refs.
* [DOCS] Added sizing tips & link to quantatative sizing video.
