SDFAB-516 Initial Sphinx framework for SD-Fabric docs

Change-Id: Icfc6caaaf6c79f4b590d5d20983906a64642ece2
diff --git a/requirements.txt b/requirements.txt
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+Sphinx~=4.1.2
+doc8~=0.9.0
+reuse~=0.13.0
+docutils==0.16
+sphinx-rtd-theme~=0.5.2
+sphinxcontrib-spelling~=7.2.1
+sphinx-multiversion~=0.2.4