Release notes with PlaidML detail added (#3455)

doc/sphinx/source/project/release-notes.rst

 .. project/release-notes.rst:
 
+:orphan:
+
 Release Notes
 #############
 
@@ -11,21 +13,31 @@ https://github.com/NervanaSystems/ngraph/releases.
 
 This page includes additional documentation updates.
 
-We are pleased to announce the release of version |version|-doc.
+We are pleased to announce the release of version |version|.
 
 
 Core updates for |version|
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-+ Better PlaidML support
+--------------------------
+
++ PlaidML support
 + Double-buffering support
 + Constant folding
 + Support for static linking
 + Additional ops
++ Preliminary static linking support
+
+Known issues
+~~~~~~~~~~~~
+
++ No PlaidML training support
+
 
-.. Latest doc updates
-.. ~~~~~~~~~~~~~~~~~~
+Latest doc updates |version|-doc
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-.. + Add instructions how to build ``NGRAPH_PLAIDML`` backend.
++ Instructions how to build ``NGRAPH_PLAIDML`` backend added to :doc:`../buildlb`. 
++ Publishing interim version of doc navigation for updates at ngraph.ai
++ GPU validations: added 5 functional TensorFlow workloads and 4 functional ONNX workloads to :doc:`../frameworks/validated/list`.
 
 
 .. important:: Pre-releases (``-rc-0.*``) have newer features, and are less stable.  
@@ -44,7 +56,6 @@ Changelog on Previous Releases
 + Graph visualization improvements
 + Preserve control dependencies in more places
 + GetOutputElement has single input
-.. + Add instructions how to build ``NGRAPH_PLAIDML`` backend.
 
 
 0.23
@@ -89,7 +100,7 @@ Changelog on Previous Releases
 + offset arg for tensor creation is deprecated
 + static linking support
 + Initial test of 0.21-doc
-+ Updated :doc:`doc-contributor-README` for new community-based contributions. 
++ Updated ``doc-contributor-README`` for new community-based contributions. 
 + Added instructions on how to test or display the installed nGraph version.
 + Added instructions on building nGraph bridge (ngraph-bridge).
 + Updated Backend Developer Guides and ToC structure.