Reorganising documentation

Also changing to furo theme.

docs/source/getting_started/installation.rst

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+Installation
+------------
+
+Supported platforms
+~~~~~~~~~~~~~~~~~~~
+
+Source tree and build system
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The Yosys source tree is organized into the following top-level
+directories:
+
+-  | backends/
+   | This directory contains a subdirectory for each of the backend modules.
+
+-  | frontends/
+   | This directory contains a subdirectory for each of the frontend modules.
+
+-  | kernel/
+   | This directory contains all the core functionality of Yosys. This includes
+     the functions and definitions for working with the RTLIL data structures
+     (rtlil.h and rtlil.cc), the main() function (driver.cc), the internal
+     framework for generating log messages (log.h and log.cc), the internal
+     framework for registering and calling passes (register.h and register.cc),
+     some core commands that are not really passes (select.cc, show.cc, …) and a
+     couple of other small utility libraries.
+
+-  | passes/
+   | This directory contains a subdirectory for each pass or group of passes.
+     For example as of this writing the directory passes/opt/ contains the code
+     for seven passes: opt, opt_expr, opt_muxtree, opt_reduce, opt_rmdff,
+     opt_rmunused and opt_merge.
+
+-  | techlibs/
+   | This directory contains simulation models and standard implementations for
+     the cells from the internal cell library.
+
+-  | tests/
+   | This directory contains a couple of test cases. Most of the smaller tests
+     are executed automatically when make test is called. The larger tests must
+     be executed manually. Most of the larger tests require downloading external
+     HDL source code and/or external tools. The tests range from comparing
+     simulation results of the synthesized design to the original sources to
+     logic equivalence checking of entire CPU cores.
+
+The top-level Makefile includes frontends/\*/Makefile.inc,
+passes/\*/Makefile.inc and backends/\*/Makefile.inc. So when extending Yosys it
+is enough to create a new directory in frontends/, passes/ or backends/ with
+your sources and a Makefile.inc. The Yosys kernel automatically detects all
+commands linked with Yosys. So it is not needed to add additional commands to a
+central list of commands.
+
+Good starting points for reading example source code to learn how to write
+passes are passes/opt/opt_rmdff.cc and passes/opt/opt_merge.cc.
+
+See the top-level README file for a quick Getting Started guide and build
+instructions. The Yosys build is based solely on Makefiles.
+
+Users of the Qt Creator IDE can generate a QT Creator project file using make
+qtcreator. Users of the Eclipse IDE can use the "Makefile Project with Existing
+Code" project type in the Eclipse "New Project" dialog (only available after the
+CDT plugin has been installed) to create an Eclipse project in order to
+programming extensions to Yosys or just browse the Yosys code base.