Docs: Advanced tag use

Add complex pycode example, generating tasks and then parsing tags to select source files.  Warn against doing it, and reiterate the usefulness of `--dumptasks` and `--dumpcfg`.
Modify gen.sh for modifiable script/task and to echo how it was called (in case they need to be regenerated, which isn't a problem for the regular example/task1 but is for the complex one).

docs/source/advanced/index.rst

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+Advanced SBY usage/examples
+===========================
+
+.. toctree::
+
+   tags

docs/source/advanced/tags.rst

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+Tasks and tags
+==============
+
+Complex pycode blocks
+---------------------
+The following example demonstrates how to configure safety and liveness checks
+for all combinations of host and device implementations.  Note that the ``task``
+variable is set to a value of ``None`` when initially parsing the ``.sby`` file,
+so it is important to check ``if task is not None`` to avoid the
+``AttributeError`` when trying to use string methods on it.
+
+.. literalinclude:: /../examples/tags/complex.sby
+   :language: sby
+   :caption: ``docs/examples/tags/complex.sby``
+
+.. warning::
+
+   Including section headings in pycode blocks like this is a quick way to make
+   your ``.sby`` file very difficult to follow and is generally not recommended.
+   However since it is not possible to share variables between pycode sections,
+   in cases such as this it does simplify adding new host/device implementations
+   while avoiding duplication of the tag parsing.
+
+When working with complex ``.sby`` files like this, it is very helpful to use
+``--dumptasks`` and ``--dumpcfg`` to confirm things are being pre-processed as
+expected.
+
+.. literalinclude:: /../examples/tags/complex.log
+   :language: console
+   :start-at: dumptasks
+   :end-before: dumptags
+
+.. literalinclude:: /../examples/tags/complex.log
+   :language: console
+   :start-at: dumpcfg