Bug 1613526 - Create a code quality documentation and move the appropriate docs r=ahal

This for a few reasons:
* The summary becomes the landing page for code quality:
  https://firefox-source-docs.mozilla.org/tools/static-analysis/summary.html
* I don't think we need a full code quality category
* Closer to the source-code-doc
* All the files at the same place

Differential Revision: https://phabricator.services.mozilla.com/D61767

docs/code-quality/coding-style/coding_style_python.rst

@@ -0,0 +1,77 @@
+===================
+Python Coding style
+===================
+
+Coding style
+~~~~~~~~~~~~
+
+`black <https://github.com/psf/black/>`_ is the tool used to reformat the Python code.
+
+Linting
+~~~~~~~
+
+The Python linting is done by `flake8 <https://gitlab.com/pycqa/flake8>`_.
+flake8 is executed by mozlint both at review phase and in the CI.
+See the :ref:`flake8` documentation for more information.
+
+
+Indentation
+~~~~~~~~~~~
+
+Four spaces in Python code.
+
+
+Makefile/moz.build practices
+----------------------------
+
+-  Changes to makefile and moz.build variables do not require
+   build-config peer review. Any other build system changes, such as
+   adding new scripts or rules, require review from the build-config
+   team.
+-  Suffix long ``if``/``endif`` conditionals with #{ & #}, so editors
+   can display matched tokens enclosing a block of statements.
+
+   ::
+
+      ifdef CHECK_TYPE #{
+        ifneq ($(flavor var_type),recursive) #{
+          $(warning var should be expandable but detected var_type=$(flavor var_type))
+        endif #}
+      endif #}
+
+-  moz.build are python and follow normal Python style.
+-  List assignments should be written with one element per line. Align
+   closing square brace with start of variable assignment. If ordering
+   is not important, variables should be in alphabetical order.
+
+   .. code-block:: python
+
+      var += [
+          'foo',
+          'bar'
+      ]
+
+-  Use ``CONFIG['CPU_ARCH'] {=arm}`` to test for generic classes of
+   architecture rather than ``CONFIG['OS_TEST'] {=armv7}`` (re: bug 886689).
+
+
+Other advices
+~~~~~~~~~~~~~
+
+-  Install the
+   `mozext <https://hg.mozilla.org/hgcustom/version-control-tools/file/default/hgext/mozext>`__
+   Mercurial extension, and address every issue reported on commit,
+   qrefresh, or the output of ``hg critic``.
+-  Follow `PEP 8 <https://www.python.org/dev/peps/pep-0008/>`__.
+-  Do not place statements on the same line as ``if/elif/else``
+   conditionals to form a one-liner.
+-  Global vars, please avoid them at all cost.
+-  Exclude outer parenthesis from conditionals.Use
+   ``if x > 5:,``\ rather than ``if (x > 5):``
+-  Use string formatters, rather than var + str(val).
+   ``var = 'Type %s value is %d'% ('int', 5).``
+-  Utilize tools like
+   `pylint <https://pypi.python.org/pypi/pylint>`__ or
+   `pychecker <http://pychecker.sourceforge.net>`__ to screen
+   sources for common problems.
+-  Testing/Unit tests, please write them.