Docs: Section/folder for yosys source details

Move test_suites page into said folder.
Placeholder page for building with verific.

docs/source/yosys_source/build_verific.rst

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+Compiling with Verific library
+------------------------------

docs/source/yosys_source/index.rst

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+Yosys source details
+--------------------
+
+.. toctree::
+
+   build_verific
+   test_suites

docs/source/yosys_source/test_suites.rst

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+Testing Yosys
+=============
+
+.. todo:: more about the included test suite
+
+Automatic testing
+-----------------
+
+.. only:: html
+
+   The `Yosys Git repo`_ has automatic testing of builds and running of the
+   included test suite on the following platforms:
+
+   - Ubuntu |test-linux|
+   - macOS |test-macos|
+
+.. _Yosys Git repo: https://github.com/YosysHQ/yosys
+
+.. |test-linux| image:: https://github.com/YosysHQ/yosys/actions/workflows/test-linux.yml/badge.svg?branch=main
+.. |test-macos| image:: https://github.com/YosysHQ/yosys/actions/workflows/test-macos.yml/badge.svg?branch=main
+
+For up to date information, including OS versions, refer to `the git actions
+page`_.
+
+.. _the git actions page: https://github.com/YosysHQ/yosys/actions