[Docs update] Make a reference to Exposure taxonomies in Schema reference Exposure taxonomy scheme

[pzwsk]

What is the context or reason for the change?

With the revision of the standard, there is no more reference to the existing exposure taxonomies such as GED4ALL or GEM in the exposure section of the schema reference.

See https://docs.riskdatalibrary.org/en/latest/reference/schema/#exposure-metadata

What is your proposed change?

In Schema reference > Exposure metadata, I propose to add a reference to the exposure taxonomies listed in the page Other standards https://docs.riskdatalibrary.org/en/latest/rdl/other-standards/#ged4all

I would also add our preferred option which is GED4ALL.

[matamadio]

This is done in the Vulnerability schema:

Exposure "Taxonomy" item should be the same.