doc: remove instructions regarding numbered steps

"Numbered steps" were dropped a long time ago in a previous revamp of
the stylesheet, so dropping instructions related to them in the
documentation guidelines.

Also remove unecessary mention to "how to build the doc" as it's
already mentioned as a note at the top of the document.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

doc/contribute/documentation/guidelines.rst

@@ -580,29 +580,3 @@ in the Zephyr setup.  Within a tab, you can have most any content *other
 than a heading* (code-blocks, ordered and unordered lists, pictures,
 paragraphs, and such).  You can read more about sphinx-tabs from the
 link above.
-
-Instruction Steps
-*****************
-
-Also introduced in the :ref:`getting_started` is a style that makes it
-easy to create tutorial guides with clearly identified steps. Add
-the ``.. rst-class:: numbered-step`` directive immediately before a
-second-level heading (by project convention, a heading underlined with
-asterisks ``******``, and it will be displayed as a numbered step,
-sequentially numbered within the document.  For example::
-
-   .. rst-class:: numbered-step
-
-   Put your right hand in
-   **********************
-
-.. rst-class:: numbered-step
-
-Put your right hand in
-**********************
-
-See the :zephyr_raw:`doc/develop/getting_started/index.rst` source file and
-compare with the :ref:`getting_started` to see a full example.  As implemented,
-only one set of numbered steps is intended per document.
-
-For instructions on building the documentation, see :ref:`zephyr_doc`.