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)