Add an index page for guides (nginx#990)

Problem:
The doc index page (docs/README.md) doesn't include any guides

Solution:
- Add an index page for the guides
- Add a link to the guides directory from the main index page.

Testing: none

Author: miledxz

docs/README.md

@@ -20,5 +20,6 @@ container.
 
 ### Directories
 
+- [Guides](guides): Guides about configuring NGINX Kubernetes Gateway for various use cases.
 - [Developer](developer/): Docs for developers of the project. Contains guides relating to processes and workflows.
 - [Proposals](proposals/): Enhancement proposals for new features.

docs/guides/README.md

@@ -0,0 +1,10 @@
+# Guides
+
+This directory contains guides for configuring NGINX Kubernetes Gateway for various use cases.
+
+## Contents
+
+- [Securing Traffic using Let's Encrypt and Cert-Manager](integrating-cert-manager.md): Shows how to secure
+  traffic from clients to NGINX Kubernetes Gateway with TLS using Let's Encrypt and Cert-Manager.
+- [Using NGINX Kubernetes Gateway to Upgrade Applications without Downtime](upgrade-apps-without-downtime.md):
+  Explains how to use NGINX Kubernetes Gateway to upgrade applications without downtime.