Rename Development into O/S

Link to pre-built images in Armbian page

docs/armbian.md

@@ -1,9 +1,13 @@
 # Armbian
 
-Debian based Linux for ARM based single-board computers
+Debian & Ubuntu based Linux for ARM based single-board computers
 
 [https://www.armbian.com](https://www.armbian.com "Armbian")
 
+## Pre-built images
+
+You can find pre-built **Armbian** images for Helios4 on our [Download](/download) page or directly on [Armbian website](https://dl.armbian.com/helios4/). 
+
 ## How to build an image or a kernel?
 
 Supported build environment is **Ubuntu Bionic 18.04 x64** ([minimal iso image](http://archive.ubuntu.com/ubuntu/dists/bionic/main/installer-amd64/current/images/netboot/mini.iso)).

mkdocs.yml

@@ -59,8 +59,12 @@ markdown_extensions:
 pages:
 - Introduction: 'index.md'
 - Kit Assembly: 'kit.md'
-- Install: 'install.md'
 - Download: 'download.md'
+- Install: 'install.md'
+- O/S:
+  - Armbian : 'armbian.md'
+  - FreeBSD : 'freebsd.md'
+  - U-Boot (bootloader) : 'uboot.md'
 - Software:
   - Mdadm (RAID) : 'mdadm.md'
   - NextCloud : 'nextcloud.md'
@@ -76,9 +80,5 @@ pages:
   - SDIO (SD Card) : 'sdcard.md'
   - SPI (NOR Flash) : 'spi.md'
   - Wake-On-LAN : 'wol.md'
-- Development:
-  - Armbian : 'armbian.md'
-  - FreeBSD : 'freebsd.md'
-  - U-Boot : 'uboot.md'
 - Documents: 'docs.md'
 - About: 'about.md'