docs: don't repeat top-level dependencies

Now that the top-level `tox` inheritance is fixed, there's no need to
repeat the dependencies across several files.

This (partially) reverts commit 47c8962.

Change-Id: Ie60ed866b14ac7ac424ea2cea05c99d0f996f854

docs/requirements.txt

@@ -1,12 +1,7 @@
 alabaster>=0.7.12,<1
 docutils>=0.17.1,<1
-matplotlib>=3.7.3,<4
 myst-parser>=0.16.1,<1
-networkx>=3.1,<4
-numpy>=1.24.4,<2
 Pygments>=2.11.2,<3
 rstcheck
-scipy>=1.10.1,<2
-Sphinx>=5.3.0,<6
+Sphinx>=4.4.0,<5
 sphinxcontrib-bibtex>=2.4.1,<3
-xlrd>=1.2.0,<2