add DCATUS 3.0 guidance on use of "geospatial" in theme and linking a ISO/CSDGM record as distribution

## User Story

In order to offer better DCATUS 3.0 guidance, datagov wants to update resources.data.gov with information on how to store "geospatial" in theme and how to link ISO/CSDGM records as distributions.

## Acceptance Criteria

[ACs should be clearly demoable/verifiable whenever possible. Try specifying them using [BDD](https://en.wikipedia.org/wiki/Behavior-driven_development#Behavioral_specifications).]

- [ ] GIVEN how guidance on these topics help people \
WHEN resources.data.gov is updated with this guidance \
THEN guidance is available for use \
[AND optionally another verifiable outcome]

## Background
- here's an [example](https://resources.data.gov/resources/podm-field-mapping/#:~:text=Insert%20default%20value,identify%20geospatial%20resources) explanation for geospatial in theme for dcatus 1.1. we'd want to do something similar for dcatus 3.0
- we want to do a similar thing but for how to reference ISO/CSDGM records as distributions in dcatus 3.0. here's a screenshot from what FGDC shares. provided by @chris-macdermaid
- related [ticket](https://github.com/GSA/data.gov/issues/6199)

## Security Considerations ([required](https://nvd.nist.gov/800-53/Rev4/control/CM-4))

[comment]: # "Our SSP says 'The Data.gov team ensures security implications are considered as part of the agile requirements refinement process by including a section in the issue template used as a basis for new work.' so please don't remove this section without care."
[Any security concerns that might be implicated in the change. "None" is OK, just be explicit here!]

## Sketch

[Notes or a checklist reflecting our understanding of the selected approach]