Create top-level subject guides page, add example guides

[jcohen02]

We plan to include a section of the toolkit providing subject guides - an overview of a particular field where RSEs may work, providing links to key resources in the domain, details of widely used tools and processes, etc.

We need to add a top-level page for this, including a list of all the subject guides to be added. We already have an example guide for Bioinformatics.

What is the reasoning for this? Many of us have had the experience of beginning a collaboration as an RSE in a new field/domain. It's very easy to get dropped into a field you don't know about and be confident about how to write code but not know enough about the domain to be able to begin the necessary conversations with researchers/developers in the field to undertake the required work. We often don't have the vocabulary, and we don't know how to ask for it! The subject guides are there to help address these problems...