doc update

[aradhakrishnanGFDL]

Do we have docs about combining catalogs in the sphinx page? If not, we need one.

Here is a start

Here are some pointers to the combine_cats.py to combine intake_esm catalogs. 
https://github.com/NOAA-GFDL/CatalogBuilder/blob/main/catalogbuilder/scripts/combine_cats.py
Usage is described in this PR #58 (comment)
The other option is to use fre-cli. Here's Chris's documentation for that. 
Using fre-cli
module load fre/canopy
fre catalog --help

Usage: fre catalog [OPTIONS] COMMAND [ARGS]...

   - access fre catalog subcommands

Options:
  --help  Show this message and exit.

Commands:
  builder   - Generate .csv and .json files for catalog
  merge     - Merge two or more more catalogs into one
  validate  - Validate a catalog against catalog schema
c2b:~%>fre catalog merge --help 
Usage: fre catalog merge [OPTIONS]

  - Merge two or more more catalogs into one

Options:
  --input TEXT   Catalog json files to be merged, space-separated  [required]
  --output TEXT  Merged catalog  [required]
  --help         Show this message and exit.
fre catalog merge --input ~/catalog/catalog1.json --input ~/catalog/catalog2.json --output combined.json 
/home/c2b/catalog/catalog1.csv
/home/c2b/catalog/catalog2.csv
INFO: Schema differs
{'catalog_file': '/home/Chris.Blanton/catalog/catalog2.csv'}
We can combine since the catalog_file is the only difference
Combined catalog specification-  combined.json
Combined csv/catalog-  combined.csv