Sphinx doc (#581)

* New docs directory with minimal config

* Based on docs directory of rocBLAS

* Config for running Doxygen then Sphinx to generate HTML

* Add minimal content - intro to doc

* Add some boilerplate sections to doc

* content still needs to be done,
* e.g., need to generate API documentation using Doxygen
* need to write contributor guide

* Start Softmax section of Support Primitives doc

* Written as a test bed for typesetting math content

* Need to decide how much detail to go into

* add doc directories to git ignore file.

* Minor edits - new line at EOF, change year in copyright notices

* Port Markdown files to ReStructuredText

* Copy Markdown files from pre-existing doc directory to docs directory

* Convert to reStructured Text (rst) - section headings, links, tables
  have a different syntax in rst

* New rst files added to index - can generate HTML with same style as
  HTML generated from rst files in previous commits

* Intention is to make all the content in doc redundant and use rst
  throughout rather than mix of md and rst

* Extend Softmax section of Primitives Guide

* rename l to z

* add material on applying softmax row-wise to matrix

* define macro for diag operator (represents diagonal matrix)

---------

Co-authored-by: zjing14 <zhangjing14@gmail.com>

docs/source/API_Reference_Guide.rst

@@ -0,0 +1,23 @@
+
+===================
+API Reference Guide
+===================
+
+------------
+Introduction
+------------
+
+This document contains details of the APIs for the Composable Kernel (CK) library and introduces some of the key design
+principles that are used to write new classes that extend CK functionality.
+
+=================
+Using CK API
+=================
+
+This section describes how to use the CK library API.
+
+-----------------
+CK Datatypes
+-----------------
+
+[TODO]