Add how NGINX IC works doc

* Add intro section
* Add how NGINX IC works doc
* docs: copy edit

Co-authored-by: Travis Martin <[email protected]>

Author: pleshakov

docs-web/index.rst

@@ -9,7 +9,7 @@ Request your `free 30-day trial <https://www.nginx.com/free-trial-request-nginx-
 .. toctree::
    :maxdepth: 2
 
-   overview
+   intro/index
    installation/index
    configuration/index
    logging-and-monitoring/index

docs-web/intro/how-nginx-ingress-controller-works-images/control-loop.png

@@ -0,0 +1,8 @@
+Intro
+======================
+
+.. toctree::
+   :maxdepth: 2
+
+   overview
+   how-nginx-ingress-controller-works