Revert "Update README to use optional dependency over requirements.txt"

In 4ab4de3 (Update README to use optional dependency over
requirements.txt, 2022-09-06), docs/README.rst was changed to suggest
installing the requirements for the documentation build using 'pip
install .[doc] -U' instead of using docs/requirements.txt.

This works for a first build but since it is not an editable install,
any changes to the docstrings are not reflected in the generated API
section in subsequent builds, since Sphinx's autodoc extensions looks at
installed modules only.

Revert that commit, going back to suggesting 'pip install -U -r
docs/requirements.txt'. The requirements file itself consists of '-e
.[doc]' since 95de1fe (Move documentation requirements to setup.cfg,
2022-09-06) (the parent of the commit we are reverting), so this does
not change the list of installed packages.

This reverts commit 4ab4de3.

docs/README.rst

@@ -26,7 +26,7 @@ the following tools are needed to build the documentation:
 In a conda environment, or a Python 3 ``venv``, you should be able to run::
 
   cd ipython
-  pip install .[doc] -U
+  pip install -U -r docs/requirements.txt
 
 
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