competition: update video deliverable pages

• add language about English captions being automatically generated
• replace "Subtitle" by "Captions" as this is the term used in the uploads page
• update overall text to be more precise, concise and clear
• add instructions on how to submit
• add intro sequence video download
• remove "Light Stage Talk" text stating that it is 15 minutes (it's now 10m)
• update/add slack link as https://competition.igem.org/participation/connect
• add Project Promotion inspiration videos (links where broken before)