Add Offline docs to docs website's sidebar (#6251)

Fix the offline docs URL in README

README.md

@@ -96,6 +96,7 @@ vagrant up
 - [Large deployments](docs/large-deployments.md)
 - [Adding/replacing a node](docs/nodes.md)
 - [Upgrades basics](docs/upgrades.md)
+- [Air-Gap installation](docs/offline-environment.md)
 - [Roadmap](docs/roadmap.md)
 
 ## Supported Linux Distributions
@@ -143,7 +144,7 @@ Note: The list of validated [docker versions](https://kubernetes.io/docs/setup/p
 
 - **Minimum required version of Kubernetes is v1.16**
 - **Ansible v2.9+, Jinja 2.11+ and python-netaddr is installed on the machine that will run Ansible commands**
-- The target servers must have **access to the Internet** in order to pull docker images. Otherwise, additional configuration is required (See [Offline Environment](https://github.com/kubernetes-sigs/kubespray/blob/master/docs/downloads.md#offline-environment))
+- The target servers must have **access to the Internet** in order to pull docker images. Otherwise, additional configuration is required (See [Offline Environment](docs/offline-environment.md))
 - The target servers are configured to allow **IPv4 forwarding**.
 - **Your ssh key must be copied** to all the servers part of your inventory.
 - The **firewalls are not managed**, you'll need to implement your own rules the way you used to.

docs/_sidebar.md

@@ -9,6 +9,7 @@
   * [HA Mode](docs/ha-mode.md)
   * [Adding/replacing a node](docs/nodes.md)
   * [Large deployments](docs/large-deployments.md)
+  * [Air-Gap Installation](docs/offline-environment.md)
 * CNI
   * [Calico](docs/calico.md)
   * [Contiv](docs/contiv.md)