Start writing the pads guide

Author: Noarkhh

guides/useful_concepts/pads.md

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+# Everything about pads
+
+When developing some intuition about the structure of pipelines the pads are
+something that can't be ignored. If you think about elements and bins as some
+sort of containers in which some processing happens, then pads are the parts
+that with these tanks are connected with. A pad of one element can only connect
+to a single pad of other element and only once two pads are connected the media
+can flow through them.
+
+`t:Membrane.Pad.element_spec/0`