Remove links to isnat function from docs (#2250)

`isnat` function is not planning to be supported by dpnp, because it
requires an input array with datetime or timedelta data type.
The PR proposes to clean up the documentation and to remove the links to
`isnat` function.

Author: antonwolfy

doc/reference/logic.rst

@@ -12,12 +12,10 @@ Truth value testing
 
    dpnp.all
    dpnp.any
-   dpnp.in1d
-   dpnp.isin
 
 
-Infinities and NaNs
--------------------
+Array contents
+--------------
 
 .. autosummary::
    :toctree: generated/
@@ -45,7 +43,7 @@ Array type testing
    dpnp.isscalar
 
 
-Logic operations
+Logical operations
 ----------------
 
 .. autosummary::

doc/reference/ufunc.rst

@@ -161,7 +161,6 @@ Floating functions
    dpnp.isfinite
    dpnp.isinf
    dpnp.isnan
-   dpnp.isnat
    dpnp.fabs
    dpnp.signbit
    dpnp.copysign

dpnp/dpnp_iface_logic.py

@@ -1168,8 +1168,6 @@ def isfortran(a):
 :obj:`dpnp.isposinf` : Test element-wise for positive infinity,
                         return result as bool array.
 :obj:`dpnp.isfinite` : Test element-wise for finiteness.
-:obj:`dpnp.isnat` : Test element-wise for NaT (not a time)
-                    and return result as a boolean array.
 
 Examples
 --------