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.

We're going to require a newer Sphinx because some automagic version
deps bring sphinxcontrib-applehelp which now requires Sphinx v5. While
we're at it, let's bring this to Sphinx v7. This required that `lang`
change (changed in v5).

Of course, because of the version pinning hell, Sphinx v7 requires newer
myst-parser, so let's be bold and go all the way to v2. What could
possibly go wrong?

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
+myst-parser>=2,<3
 Pygments>=2.11.2,<3
 rstcheck
-scipy>=1.10.1,<2
-Sphinx>=5.3.0,<6
+Sphinx>=7.2.6,<8
 sphinxcontrib-bibtex>=2.4.1,<3
-xlrd>=1.2.0,<2