Document `stdJson` and JSON std-cheats (Forge-Std)

See https://github.com/foundry-rs/forge-std/pull/138

Note that there are already some references to `stdJson` and JSON std-cheats in the [`parseJson` docs](https://book.getfoundry.sh/cheatcodes/parse-json).

Also, the list of Forge-Std contracts/libraries needs to be updated to include StdJson:
- in [overview](https://book.getfoundry.sh/forge/forge-std#standard-libraries)
- in [reference](https://book.getfoundry.sh/reference/forge-std/#forge-stds-test)

Start by reading Forge-Std PR #138 and the existing parseJson documentation at book.getfoundry.sh/cheatcodes/parse-json. Update the Forge-Std overview and reference sections to document stdJson and the JSON std-cheats, and include StdJson in both listed contract/library inventories.

Written by the indexing model from the issue text.