Update the Monarch KG Tools documentation to have a landing page

[sagehrke]

Hi @ptgolden,
Can we please add a landing/home page to the upgraded Monarch KG tools documentation site?
For now, it can be something simple like this (please edit / get feedback from others...much of this is copied from the contributing.md):

Welcome to the Monarch Knowledge Graph and Tools Documentation

The Monarch Knowledge Graph (Monarch KG) and Tools is an extensive ecosystem made for the benefit of clinicians, researchers, and scientists. The knowledge graph consists of millions of entities – genes, diseases, phenotypes, and many more – imported from dozens of sources.

• Learn more about how to contribute here
• Submit a ticket with questions or requests for support in our Monarch-app repository