Add text to Mapping Header file

[emhaston]

Add a link to https://github.com/tdwg/mids/tree/main/source/mappings for documentation on the Mappings page

Would it make sense to include more markdown at the top of the page, similar to the Information Elements page? This would allow us to link to other resources and put some of the text from here https://github.com/tdwg/mids/blob/main/source/mappings/README.md

As a trial, I've copied across the text from https://github.com/tdwg/mids/edit/main/source/mappings/README.md to the mappings header source file here https://github.com/tdwg/mids/blob/main/source/md/mappings-header.md.

The text will need to be checked and updated where appropriate.

The other option would be to have this text in a separate webpage on the website.