Update doc (#1003)

* doc: update makefile and build scripts

* actions: add workflow 'doc'

* doc: reorganize sections

* doc: fix 'unknown option' warnings, headings, spaces, etc.

* doc: add subdir 'examples', move 'quick_start' sources

* doc: add section 'Development/Debugging'

* doc: add section'Development/Synthesis'

* doc: update roadmap

* doc: add section examples

* doc: use standard domain

* doc: add comment about 'vhd' vs 'vhdl'

2 files changed, 69 insertions, 0 deletions

diff --git a/doc/examples/quick_start/hello/README.rst b/doc/examples/quick_start/hello/README.rst
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+++ b/doc/examples/quick_start/hello/README.rst
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+.. _QuickStart:hello:
+
+`Hello world` program
+=====================
+
+To illustrate the general purpose of `VHDL`, the following block is a commented `Hello world` program which is saved in
+a file named :file:`hello.vhdl`:
+
+.. literalinclude:: hello.vhdl
+   :language: vhdl
+
+.. TIP::
+
+   * Both ``.vhdl`` and ``.vhd`` extensions are used for `VHDL` source files, while ``.v`` is used for Verilog.
+
+      * Since, extension ``.vhd`` is also interpreted as a `Virtual Hard Disk <https://en.wikipedia.org/wiki/VHD_(file_format)>`_
+        file format, some users prefer ``.vhdl``, to avoid ambiguity. This is the case with `GHDL`'s codebase. However, in order
+        to maintain `backward-compatibility <https://en.wikipedia.org/wiki/8.3_filename>`_ with legacy DOS systems,
+        other users prefer ``.vhd``.
+
+   * Unless you use especial characters, either `UTF-8` or `ISO-8859-1` encodings can be used. However, if you do, the
+     latter should be used. The standard defines ASCII (7-bit encoding) or ISO Latin-1 (ISO-8859-1) as default.
+     However, GHDL has a relaxing option, :option:`--mb-comments` (multi byte), to allow UTF-8 or other encodings in
+     comments.
+
+- First, you have to compile the file; this is called :ref:`analysis <Analysis:command>` of a design file in `VHDL`
+  terms. Run ``ghdl -a hello.vhdl`` in the `shell`. This command creates or updates a file :file:`work-obj93.cf`, which
+  describes the library ``work``.
+- Then, run ``ghdl -e hello_world`` in the `shell`. Command :option:`-e` means :ref:`elaborate <Elaboration:command>`,
+  which is used to build a design, with the ``hello_world`` entity at the top of the hierarchy.
+- Last, you can directly launch the simulation :ref:`running <Run:command>` ``ghdl -r hello_world`` in the `shell`. The
+  result of the simulation will be shown on screen:
+
+.. code-block:: shell
+
+  Hello world!
+
+.. HINT::
+   If a GCC/LLVM variant of `GHDL` is used:
+
+   * :ref:`Analysis <Analysis:command>` generates a file, :file:`hello.o`, which is the object file corresponding to
+     your `VHDL` program. This is not created with :ref:`mcode <BUILD>`. These kind of object files can be
+     compiled into foreign programs (see :ref:`Linking_with_Ada`).
+   * The :ref:`elaboration <Elaboration:command>` step is mandatory after running the analysis and prior to launching the
+     simulation. This will generate an executable binary named :file:`hello_world`.
+   * As a result, :option:`-r` is just a passthrough to the binary generated in the `elaboration`. Therefore, the
+     executable can be run directly: ``./hello_world``. See :option:`-r` for more informartion.
+
+.. HINT::
+
+   :option:`-e` can be bypassed with :ref:`mcode <BUILD>`, since :option:`-r` actually elaborates the design and saves
+   it on memory before running the simulation. But you can still use it to check for some elaboration problems.
diff --git a/doc/examples/quick_start/hello/hello.vhdl b/doc/examples/quick_start/hello/hello.vhdl
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+--  Hello world program
+use std.textio.all; -- Imports the standard textio package.
+
+--  Defines a design entity, without any ports.
+entity hello_world is
+end hello_world;
+
+architecture behaviour of hello_world is
+begin
+  process
+    variable l : line;
+  begin
+    write (l, String'("Hello world!"));
+    writeline (output, l);
+    wait;
+  end process;
+end behaviour;