PR: Transform constants.md to rst format

spec/API_specification/constants.rst

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+Constants
+=========
+
+    Array API specification for constants.
+
+A conforming implementation of the array API standard must provide and support the following constants adhering to the following conventions.
+
+-   Each constant must have a Python floating-point data type (i.e., ``float``) and be provided as a Python scalar value.
+
+Objects in API
+--------------
+
+.. currentmodule:: signatures.constants
+
+..
+  NOTE: please keep the functions in alphabetical order
+
+.. autosummary::
+   :toctree: generated
+
+   e
+   inf
+   nan
+   pi

spec/API_specification/signatures/constants.py

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+e = 2.718281828459045
+"""
+IEEE 754 floating-point representation of Euler's constant.
+
+``e = 2.71828182845904523536028747135266249775724709369995...``
+"""
+
+inf = float('inf')
+"""
+IEEE 754 floating-point representation of (positive) infinity.
+"""
+
+nan = float('nan')
+"""
+IEEE 754 floating-point representation of Not a Number (``NaN``).
+"""
+
+pi = 3.141592653589793
+"""
+IEEE 754 floating-point representation of the mathematical constant ``π``.
+
+``pi = 3.1415926535897932384626433...``
+"""
+
+__all__ = ['e', 'inf', 'nan', 'pi']