[docs] Include `@field` example under sqlc.arg

[Kangaroux]

https://docs.sqlc.dev/en/latest/reference/macros.html
https://docs.sqlc.dev/en/latest/howto/named_parameters.html

The docs mention

See more examples in Naming parameters.

It would be nice to include an inline example on the macros page, I wasn't aware this syntax existed.

Might also be good to just take inventory on what's documented in the howto pages vs. reference pages, ideally the reference should have everything the howto does