Revisit instructions to install Anaconda

[rebkwok]

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

[evansd]

Do we need to install in a venv? By design, opensafely-cli has no dependencies so it can safely installed system-wide (or user wide) without fear of things breaking. I think creating the venv and then having to remember to activate it each time potentially adds friction.