[MLIR][OpenMP][Docs] NFC: Reorganize 'omp' dialect documentation

mlir/docs/Dialects/OpenMPDialect/ODS.md

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+# ODS Documentation
+
+[include "Dialects/OpenMPDialect.md"]

mlir/docs/Dialects/OpenMPDialect/_index.md

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+# 'omp' Dialect
+
+The `omp` dialect is for representing directives, clauses and other definitions
+of the [OpenMP programming model](https://www.openmp.org). This directive-based
+programming model, defined for the C, C++ and Fortran programming languages,
+provides abstractions to simplify the development of parallel and accelerated
+programs. All versions of the OpenMP specification can be found
+[here](https://www.openmp.org/specifications/).
+
+Operations in this MLIR dialect generally correspond to a single OpenMP
+directive, taking arguments that represent their supported clauses, though this
+is not always the case. For a detailed information of operations, types and
+other definitions in this dialect, refer to the automatically-generated
+[ODS Documentation](ODS.md).
+
+[TOC]