Add maxLength property to the docs page

[rahul-snowplow]

Hi Team,

Can we please add the maxLength property to the canonical fields docs page?
This will help customers understand the failure reasons and max length allowed for canonical properties.

https://docs.snowplow.io/docs/understanding-your-pipeline/canonical-event/
https://github.com/snowplow/iglu-central/blob/master/schemas/com.snowplowanalytics.snowplow/atomic/jsonschema/1-0-0