Fix sphinx build warnings

[pboushy]

This PR resolves all the below warnings experienced during make html:

/Users/pboushy/excl/git-repos/openfl/docs/about/features.rst:6: WARNING: Duplicate explicit target name: "for more info".
/Users/pboushy/excl/git-repos/openfl/docs/about/features.rst:6: WARNING: Duplicate explicit target name: "for more info".
/Users/pboushy/excl/git-repos/openfl/docs/about/features.rst:6: WARNING: Duplicate explicit target name: "for more info".
/Users/pboushy/excl/git-repos/openfl/docs/about/features.rst:6: WARNING: Duplicate explicit target name: "for more info".
/Users/pboushy/excl/git-repos/openfl/docs/about/releases.md:24: WARNING: duplicate label about/releases:highlights, other instance in /Users/pboushy/excl/git-repos/openfl/docs/about/releases.md
/Users/pboushy/excl/git-repos/openfl/docs/developer_guide/experimental_features.rst:25: WARNING: toctree contains reference to nonexisting document 'developer_guide/workflow_interface'
/Users/pboushy/excl/git-repos/openfl/docs/get_started/examples.rst:42: WARNING: Title overline too short.

-------------------------
Workflow Interface (Experimental)
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/Users/pboushy/excl/git-repos/openfl/docs/get_started/examples.rst:42: WARNING: Title overline too short.

-------------------------
Workflow Interface (Experimental)
-------------------------
looking for now-outdated files... none found
pickling environment... done
checking consistency... /Users/pboushy/excl/git-repos/openfl/docs/README.md: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/about/features_index/pynative.rst: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/about/overview.how_can_intel_protect_federated_learning.rst: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/about/overview.what_is_intel_federated_learning.rst: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/developer_guide/running_the_federation.singularity.rst: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/get_started/install.singularity.rst: WARNING: document isn't included in any toctree
/Users/pboushy/excl/git-repos/openfl/docs/structurizer_dsl/README.md: WARNING: document isn't included in any toctree

NOTE - please double check the exclusion of README.md files from conf.py - they do not appear to be necessary for the website, but I was not certain.

[psfoley]

Thanks so much for the fixes, @pboushy. I'll test this locally in the next day and approve if everything looks good.