admin/doc-requirements.txt: pin Sphinx at 3.5.4

admin/doc-requirements.txt: pin Sphinx at 3.5.4

* pin Sphinx at 3.5.4
* pin docutils at 0.18

at least the combination of these two versions
is known to compile.

to address the bug reported at
https://sourceforge.net/p/docutils/bugs/431/

the backtrace looks like:

/home/jenkins-build/build/workspace/ceph-pr-docs/build-doc/virtualenv/lib/python3.8/site-packages/sphinx/util/docutils.py:285:
RemovedInSphinx30Warning: function based directive support is now
deprecated. Use class based directive instead.
  warnings.warn('function based directive support is now deprecated. '

Exception occurred:
  File
"/home/jenkins-build/build/workspace/ceph-pr-docs/build-doc/virtualenv/lib/python3.8/site-packages/docutils/writers/html5_polyglot/__init__.py",
line 445, in section_title_tags
    if (ids and self.settings.section_self_link
AttributeError: 'Values' object has no attribute 'section_self_link'

please note this change is not cherry-picked from
master, because master already bumped Sphinx to 3.5.4
in 4968baa2523bd2a5ca6be147b26bc28906a864c9.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>