Revisit instructions to install Anaconda

Anaconda's licensing has changed so it's no longer available for commercial use:
https://www.anaconda.com/blog/anaconda-commercial-edition-faq

There is a free tier for academics, students and hobbyists, which many of our users probably fall into.

However, given that some might not, we may want to revisit the Windows installation instructions that tell you to download Anaconda as the first step and at least provide an alternative.

Python on windows (confirmed with 3.9) comes with pip and venv, so instructions for using venv and installing opensafely without Anaconda are quite straightforward.

c:> py -m venv my-venv
c:> my-venv\Scripts\activate
(my-venv) c:> pip install opensafely

Somewhat related to #485

Start with the Windows installation instructions in the getting-started guide, especially step 3, and compare them with the linked Python venv guidance. Update the documentation to provide a non-Anaconda installation path and account for Anaconda's licensing change; done means users can install OpenSAFELY on Windows without relying on Anaconda.