Reorganising documentation

Also changing to furo theme.

docs/source/yosys_internals/flow/control_and_data.rst

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+Control and data flow
+=====================
+
+.. TODO: copypaste
+
+The data- and control-flow of a typical synthesis tool is very similar to the
+data- and control-flow of a typical compiler: different subsystems are called in
+a predetermined order, each consuming the data generated by the last subsystem
+and generating the data for the next subsystem (see :numref:`Fig. %s
+<fig:approach_flow>`).
+
+.. figure:: ../../../images/approach_flow.*
+	:class: width-helper
+	:name: fig:approach_flow
+
+	General data- and control-flow of a synthesis tool
+
+The first subsystem to be called is usually called a frontend. It does not
+process the data generated by another subsystem but instead reads the user
+input—in the case of a HDL synthesis tool, the behavioural HDL code.
+
+The subsystems that consume data from previous subsystems and produce data for
+the next subsystems (usually in the same or a similar format) are called passes.
+
+The last subsystem that is executed transforms the data generated by the last
+pass into a suitable output format and writes it to a disk file. This subsystem
+is usually called the backend.
+
+In Yosys all frontends, passes and backends are directly available as commands
+in the synthesis script. Thus the user can easily create a custom synthesis flow
+just by calling passes in the right order in a synthesis script.