doc/user: lower case json parser output

[joacoc]

Motivation

This PR lowercases the output from the parser.

Before:

CREATE MATERIALIZED VIEW my_view AS SELECT 
    json_column->>'anonymousId' AS anonymousId,
    json_column->'context'->>'userAgent' AS context_userAgent,
    json_column->'context'->>'Mixpanel' AS context_Mixpanel,
    ...

Now:

CREATE MATERIALIZED VIEW my_view AS SELECT 
    json_column->>'anonymousId' AS anonymousid,
    json_column->'context'->>'userAgent' AS context_useragent,
    json_column->'context'->>'Mixpanel' AS context_mixpanel,
    ...

[joacoc]

Additional: Should we also turn camelCase/kebab-case to snake_case?