[SPIR-V][DOC] Add SPV_INTEL_sigmoid extension specification

sycl/doc/design/spirv-extensions/SPV_INTEL_sigmoid.asciidoc

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+:extension_name: SPV_INTEL_sigmoid
+:capability_name: SigmoidINTEL
+:capability_token: 6167
+:instruction_name: OpFSigmoidINTEL
+:instruction_token: 6168
+
+= {extension_name}
+
+== Name Strings
+
+{extension_name}
+
+== Contact
+
+To report problems with this extension, please open a new issue at:
+
+https://github.com/intel/llvm
+
+== Contributors
+
+* Ben Ashbaugh, Intel
+* Victor Mustya, Intel
+
+== Notice
+
+Copyright (c) 2025 Intel Corporation.  All rights reserved.
+
+== Status
+
+* Working Draft
+
+This is a preview extension specification, intended to provide early access to
+a feature for review and community feedback. When the feature matures, this
+specification may be released as a formal extension.
+
+Because the interfaces defined by this specification are not final and are
+subject to change they are not intended to be used by shipping software
+products. If you are interested in using this feature in your software product,
+please let us know!
+
+== Version
+
+[width="40%",cols="25,25"]
+|========================================
+| Last Modified Date | 2025-10-29
+| Revision           | 1
+|========================================
+
+== Dependencies
+
+This extension is written against the SPIR-V Specification, Version 1.6 Revision 6.
+
+This extension requires SPIR-V 1.0.
+
+== Overview
+
+This extension adds instruction computing the sigmoid function as follows: *sigma*(x) = 1/(1 + e^-x^)
+
+== Extension Name
+
+To use this extension within a SPIR-V module, the following
+*OpExtension* must be present in the module:
+
+[subs="attributes"]
+----
+OpExtension "{extension_name}"
+----
+
+== New Capabilities
+
+This extension introduces new capabilities:
+
+[subs="attributes"]
+----
+{capability_name}
+----
+
+== New Instructions
+
+Instructions added under the *{capability_name}* capability:
+
+[subs="attributes"]
+----
+{instruction_name}
+----
+
+== Modifications to the SPIR-V Specification, Version 1.6
+
+=== Capabilities
+
+Modify Section 3.31, "Capability", adding these rows to the Capability table:
+
+--
+[cols="^.^2,16,15",options="header"]
+|====
+2+^.^| Capability | Implicitly Declares
+| {capability_token} | *{capability_name}* |
+|====
+--
+
+=== Instructions
+
+Add to Section 3.42.13, Arithmetic Instructions:
+
+[cols="1,1,3*3",width="100%"]
+|=====
+4+|[[{instruction_name}]]*{instruction_name}* +
+ +
+Compute sigmoid function of _Value_. +
+ +
+_Result Type_ must be a scalar or vector of
+https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#Floating[floating-point type].
+The component width must be 16 or 32 bits and must not have a Floating Point
+Encoding operand. +
+ +
+The type of _Value_ must be the same as _Result Type_. +
+ +
+Results are computed per component. +
+ +
+1+|Capability: +
+*{capability_name}*
+1+| 4 | {instruction_token}
+| _<id>_ +
+_Result Type_
+| _Result <id>_
+| _<id>_ +
+_Value_
+|=====
+
+== Issues
+
+None.
+
+== Revision History
+
+[cols="5,15,15,70"]
+[grid="rows"]
+[options="header"]
+|========================================
+|Rev|Date|Author|Changes
+|1|2025-10-29|Victor Mustya|*Initial revision*
+|========================================