Add getting started guide for using pyODK in Google CoLab

I've run into a number of people who use Colab as their Python environment and struggle to get everything setup. Perhaps we could add some more step-by-step docs for these users?

I think the tricky bits are that they need to install pyodk and they need to persist their credentials. The below screenshot is how I set up things for myself and it very helpful to a customer. Couple of things to note

* I should have probably used ! instead of % to install pyodk
* `Colab Notebooks` is the default location that Colab saves notebooks to, so that's where I stored the credentials.
* The credentials are not hidden because how to edit hidden files require more explanation.


I'm not sure if this doc should live in pyODK or the docs site. @lognaturel, any thoughts on this?

Review the existing pyODK documentation and the separate docs site to decide where this guide belongs. Start by checking how installation and credential setup are currently documented, then add a step-by-step Google Colab guide covering pyodk installation and credential persistence. Done means a Colab user can follow the guide from setup through using pyODK.

Written by the indexing model from the issue text.