Remove metadata md files in favor of redirecting to schema documenation

[aclum]

I propose deleting everything in the folder https://github.com/microbiomedata/NMDC_documentation/tree/main/docs/reference/metadata in favor of redirecting to the schema autogenerated documentation (https://microbiomedata.github.io/nmdc-schema/)? Direct links to these markdown files in reads the docs don't exist any more but the links still work and the documentation is not being kept up to date.

Before we do this we'd have to remove references to these in https://github.com/microbiomedata/NMDC_documentation/blob/main/docs/howto_guides/api_gui.md
and identify if there are any other references to this folder.

cc @turbomam @ljohnson09