docs: Fix build deps and Python Bindings page.

- In order to be able to build the documentation, the package
`sphinxcontrib-katex` also needs to be installed via pip.

- According to the matplotlib documentation [1], the second argument
to `imshow` needs to be a string describing a colormap name or an
instance of a ColorMap. An error occurred with the previously used
argument as it did not describe a defined colormap. As the argument is
optional, it can just be omitted (or replaced according to personal
style preferences).
- The relative link to the Quickstart page did not work, it has been
replaced by a doc-link according to the sphinx documentation [2].

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