[E&A] Creates Elastic Inference section (#692)

## Overview

This PR creates an `Elastic Inference` section in Explore and Analyze
and moves the `inference-api.md` and `/inference-api` content from
`Solutions/Search/Ai-search` under this section. It will contain the EIS
documentation, too.

##
[Preview](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/692/explore-analyze/elastic-inference)

Author: szabosteve

explore-analyze/elastic-inference.md

@@ -0,0 +1,14 @@
+---
+applies_to:
+  stack: ga
+  serverless: ga
+navigation_title: Elastic Inference
+---
+
+# Elastic {{infer-cap}}
+
+There are several ways to perform {{infer}} in the {{stack}}. This page provides a brief overview of the different methods:
+
+* [Using EIS (Elastic Inference Service)](elastic-inference/eis.md)
+* [Using the {{infer}} API](elastic-inference/inference-api.md)
+* [Trained models deployed in your cluster](machine-learning/nlp/ml-nlp-overview.md)

explore-analyze/elastic-inference/eis.md

@@ -0,0 +1,10 @@
+---
+applies_to:
+  stack: ga
+  serverless: ga
+navigation_title: Elastic Inference Service (EIS)
+---
+
+# Elastic {{infer-cap}} Service
+
+This is the documentation of the Elastic Inference Service.