fix(gocloud): parse gcs.usejsonapi as boolean

[fallintoplace]

Summary

ParseGCSConfig used to enable JSON reads on gcs.usejsonapi whenever the key existed, so values like "false" still switched it on.

This change now parses the property as a boolean using strconv.ParseBool and only enables JSON reads when it resolves to true.

Also included:

• Unit tests covering true, false, absent, and invalid values (not-a-bool).
• Documentation update in website/src/configuration.md.
• Removed outdated comment in io/config.go that implied truthy-by-presence behavior.

Validation

• go test ./io/gocloud -run TestParseGCSConfigUseJSONAPI -count=1

[zeroshade]

LGTM. Parsing gcs.usejsonapi via strconv.ParseBool preserves the disabled-by-default behavior and matches how other boolean properties are handled; tests cover unset/true/false/invalid.