Update documentation on electric utility/rate coverage

[eringold]

#312 updated the electric utility mapping in ComStock.

The highlighted claims are no longer accurate and need updating:

[eringold]

Some quick Athena checks:

SELECT 
    -- Calculate the percentage of weighted sqft where electric_utility_eia_code is null
    (SUM(CASE WHEN "in.electric_utility_eia_code" IS NULL 
              THEN "calc.weighted.sqft..ft2" 
              ELSE 0 END) * 100.0) / SUM("calc.weighted.sqft..ft2") AS percent_null_electric_utility
FROM "comstock_amy2018_r2_2025_md_by_state_and_county_parquet"
--> percent_null_electric_utility = 0.013159565201287936

SELECT 
    (SUM(CASE WHEN "out.utility_bills.electricity_bill_median_low_label" = 'state_average_rate'
              THEN "calc.weighted.sqft..ft2"
              ELSE 0 END) * 100.0) / SUM("calc.weighted.sqft..ft2") AS percent_state_avg_electric_bills
FROM "comstock_amy2018_r2_2025_md_by_state_and_county_parquet"
--> percent_state_avg_electric_bills = 16.823539649575324

SELECT 
    COUNT(DISTINCT "in.nhgis_tract_gisjoin") AS unique_tract_count_with_null_utility
FROM "comstock_amy2018_r2_2025_md_by_state_and_county_parquet"
WHERE "in.electric_utility_eia_code" IS NULL;
--> unique_tract_count_with_null_utility = 26