[Nexthop] Added documentation for materialized_JSON static test configs

[marif-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

There was no information about materialized_JSON static test configs in the FBOSS OSS documentation, so added a short description of these files.

Test Plan

N/A (Documentation only change)