power isa 3.1 spec added

Signed-off-by: Toshaan Bharvani <toshaan@vantosh.com>

content/specifications/powerisa31.md

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+---
+title: "POWER Instruction Set Architecture 3.1"
+date: 2021-08-30
+draft: false
+---
+
+The Power Instruction Set Architecture (ISA) Version 3.1 is a specification that describes the architecture used for the IBM POWER10 processor.  
+It defines the instructions the processors execute.  
+It is comprised of three books and a set of appendices.  
+- Book I
+  - Power ISA User Instruction Set Architecture, covers the base instruction set and related facilities available to the application programmer.
+- Book II
+  - Power ISA Virtual Environment Architecture, defines the storage model and related instructions and facilities available to the application programmer.
+- Book III
+  - Power ISA Operating Environment Architecture, defines the supervisor instructions and related facilities.

data/specifications/powerisa31.yaml

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+versions:
+  - number: "3.1"
+    date: "2020-05-01"
+    download: "https://files.openpower.foundation/s/bo728kgiWfgMHAr"
+    description: "Initial release."