Develop a good frontpage for the RDA Mappings website

[matentzn]

@llehtsalu this is a starting point for the front page.

Task: Create a markdown page which introduces the effort, its scope and lists some basic content prepared for users.

Subtasks:

• Create a markdown page (index.md) and add it into this directory with a pull request: https://github.com/mapping-commons/rda-fair-mappings/tree/main/docs (page might already be there).
• Make sure it is referenced in https://github.com/mapping-commons/rda-fair-mappings/blob/main/mkdocs.yml
• Add some useful content in there (ideally one or two logos as well to not be too dull) that paints an engaging picture of what we do.
• Make a pull request here for review.

[matentzn]

I created a draft page here:
https://github.com/mapping-commons/rda-fair-mappings/edit/main/docs/index.md

That looks like this:
https://mapping-commons.github.io/rda-fair-mappings/