doc: update Kernel documentation build system
Update the documentation build system according to Linux v5.11-rc1.
Deactive the automarkup.py extension module which on Gitlab CI is
incompatible with Unicode.
With this patch we can build the HTML documentation using either of
Sphinx 2 and Sphinx 3.
Signed-off-by:
Heinrich Schuchardt <xypron.glpk@gmx.de>
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