docs(manifest): more simple introduction to manifest reference (#4601)

Author: lucascolley

docs/reference/pixi_manifest.md

@@ -1,5 +1,6 @@
 
 The `pixi.toml` is the workspace manifest, also known as the Pixi workspace configuration file.
+It specifies environments for a workspace, and the package dependency requirements for those environments. It can also specify tasks which can run in those environments, as well as many other configuration options.
 
 A `toml` file is structured in different tables.
 This document will explain the usage of the different tables.