doc: guidelines: fix dead links

For references to files that are in the Zephyr GitHub tree, a special

role can be used that creates a hyperlink to that file.  For example a

reference to the reST file used to create this document can be generated

using ``:zephyr_file:`doc/contribute/documentation/index.rst```  that will

show up as :zephyr_file:`doc/contribute/documentation/index.rst`, a link to

using ``:zephyr_file:`doc/contribute/documentation/guidelines.rst```  that will

show up as :zephyr_file:`doc/contribute/documentation/guidelines.rst`, a link to

the "blob" file in the github repo.  There's also a

``:zephyr_raw:`doc/guides/documentation/index.rst``` role that will

``:zephyr_raw:`doc/contribute/documentation/guidelines.rst``` role that will

link to the "raw" content,

:zephyr_raw:`doc/contribute/documentation/index.rst`. (You can click on

:zephyr_raw:`doc/contribute/documentation/guidelines.rst`. (You can click on

these links to see the difference.)

.. _internal-linking: