Create a new page to explain schema groups

[rmseifried]

Issue

There is some confusion about how individual fields in the schema groups relate to each other.

For example, we agreed in #30 that we want a new page that discusses the differences between these "Spatial" fields:

Spatial Coverage (S) Geometry (S) Bounding Box (S) Centroid

And we agreed in #21 to link "Relations" to the overview at New elements for item relations and see if that section needs enhancements.

Suggested fix

1. Create a new page (under Resources for now) that explains our rationale for each of the groups and how the fields relate to each other.
2. Link each section to the schema group headings on the schema page

[kgjenkins]

Here's the pages we definitely want to have a group page for:

• [ ] Categories (Marc)
• [x] Temporal (Becky)
• [x] Spatial (Keith)
• [x] Relations
• [ ] Rights

Marc and Keith propose having separate pages for each, in the ogm-aardvark, named like group-categories.md

[kgjenkins]

I created a schema-groups branch, which has a new file group-spatial.md -- feel free to edit that branch directly if you like.

Also, we'll probably want to have a relatively consistent structure to these pages, but for now it might be good to have a variety of styles, and see what works best.

[rmseifried]

I added a new file group-temporal.md directly to the schema-groups branch.

I deliberately structured it differently (per @kgjenkins recommendation) to give us some style flavor to choose from!