Docs: changes from JF

`yosys-witness` prereq `click`.
Yosys environment vars & `yosys --help` output.
Removing Ubuntu/macOS version numbers/names.
Hide `troubleshooting` page.

docs/source/appendix/auxprogs.rst

@@ -57,3 +57,7 @@ independent of the solver.
 
 .. literalinclude:: /temp/yosys-witness
     :start-at: Usage
+
+.. note:: ``yosys-witness`` requires `click`_ Python package for use.
+
+.. _click: https://pypi.org/project/click/

docs/source/appendix/env_vars.rst

@@ -0,0 +1,31 @@
+Yosys environment variables
+===========================
+
+``HOME``
+   Yosys command history is stored in ``$HOME/.yosys_history``.  Graphics (from
+   :cmd:ref:`show` and :cmd:ref:`viz` commands) will output to this directory by
+   default.  This environment variable is also used in some cases for resolving
+   filenames with ``~``.
+
+``PATH``
+   May be used in OpenBSD builds for finding the location of Yosys executable.
+
+``TMPDIR``
+   Used for storing temporary files.
+
+``ABC``
+   When compiling Yosys with out-of-tree ABC using ``ABCEXTERNAL``, this
+   variable can be used to override the external ABC executable.
+
+``YOSYS_NOVERIFIC``
+   If Yosys was built with Verific, this environment variable can be used to
+   temporarily disable Verific support.
+
+``YOSYS_COVER_DIR`` and ``YOSYS_COVER_FILE``
+   When using code coverage, these environment variables control the output file
+   name/location.
+
+``YOSYS_ABORT_ON_LOG_ERROR``
+   Can be used for debugging Yosys internals.  Setting it to 1 causes abort() to
+   be called when Yosys terminates with an error message.
+