[Fleet] Add links to OpenAPI spec files and online documentation (#94122

)

## Summary

Added screenshots of two online viewers and links to the sites. 

<img width="934" alt="Screen Shot 2021-03-09 at 12 35 02 PM" src="https://user-images.githubusercontent.com/57655/110513158-eb5b0e00-80d3-11eb-82c4-de21c494589f.png">


### Checklist

- [x] [Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html) was added for features that require explanation or tutorials

x-pack/plugins/fleet/common/openapi/README.md

@@ -1,7 +1,19 @@
+# OpenAPI
+
+The current self-contained spec file is [as JSON](https://raw.githubusercontent.com/elastic/kibana/master/x-pack/plugins/fleet/common/openapi/bundled.json) or [as YAML](https://raw.githubusercontent.com/elastic/kibana/master/x-pack/plugins/fleet/common/openapi/bundled.yaml) and can be used for online tools like those found at https://openapi.tools/
+
+For example, online viewers for the specification like these:
+
+| <a href="https://mrin9.github.io/OpenAPI-Viewer/#/load/https%3A%2F%2Fraw.githubusercontent.com%2Felastic%2Fkibana%2Fmaster%2Fx-pack%2Fplugins%2Ffleet%2Fcommon%2Fopenapi%2Fbundled.json">View spec using MrinDoc</a>  |  <a href="https://petstore.swagger.io/?url=https://raw.githubusercontent.com/elastic/kibana/master/x-pack/plugins/fleet/common/openapi/bundled.json">View spec using Swagger UI</a> |
+|----|----|
+| <img alt="Screen Shot 2021-03-09 at 10 14 52 AM" src="https://user-images.githubusercontent.com/57655/110493024-8944dd80-80c0-11eb-97b2-0666fcca3b09.png">  | <img alt="Screen Shot 2021-03-09 at 10 14 04 AM" src="https://user-images.githubusercontent.com/57655/110493019-88ac4700-80c0-11eb-982b-d5d352143003.png"> |
+
+
 ## The `openapi` folder
 
 * `entrypoint.yaml` is the overview file which links to the various files on disk.
-* `bundled.{yaml,json}` is the resolved output of that entry & other files in a single file. It's currently generated with:
+* `bundled.{yaml,json}` is the resolved output of that entry & other files in a single file. Typically the best choice as input to tools
+  * It's currently generated with:
 
     ```
     npx swagger-cli bundle -o bundled.json -t json entrypoint.yaml
@@ -10,5 +22,4 @@
 * [Paths](paths/README.md): this defines each endpoint.  A path can have one operation per http method.
 * [Components](components/README.md): Reusable components like [`schemas`](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#schemaObject),
   [`responses`](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#responseObject)
-  [`parameters`](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#parameterObject), etc
-
+  [`parameters`](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#parameterObject), e