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/Makefile

@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line.
+SPHINXOPTS    =
+SPHINXBUILD   = sphinx-build
+SPHINXPROJ    = CK
+SOURCEDIR     = .
+BUILDDIR      = _build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)