Adding doc system to build. Updated docs, fixed links.

doc/install/Release-Numbering.md

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+### Mattermost Release Numbering 
+
+Mattermost numbers its stable releases based on the following format: 
+
+  `[Version Number].[Major Build Number].[Minor Build Number]`
+
+Version Number:
+
+- Indicates a major system release (e.g. 1.x.x, 2.x.x)
+
+Major Build Number:
+
+- Indicates significant new functionality, (e.g. 0.5.x, 0.6.x, 0.7.x)
+
+Minor Build Number:
+
+- Indicates bugfix/security releases (e.g. 1.2.5, 1.2.6)