Minecraft wiki entry for title, associated text: In Java Edition:
/title @a subtitle {"text":"The story begins...","color":"gray","italic":true}
/title @a title {"text":"Chapter I","bold":true}
In Bedrock Edition using plain text:
/title @a subtitle §7§oThe story begins...
/title @a title §lChapter I
In Bedrock Edition with raw text variants:
/titleraw @a subtitle {"rawtext": [{"text":"§7§oThe story begins..."}]}
/titleraw @a title {"rawtext": [{"translate":"§lChapter %%s","with":["I"]}]}