Changes to Step 2. Create or Obtain a Video Transcript (fixes DOC-1143)

@mslotkeEDX this changes "strongly recommended" to "required". I also did some reorganization and tweaking to this Step 2 section.
@srpearce , let me know if I introduced any errors!
@mhoeber , @catong , FYI
Nice to have this recommended --> required change in ASAP to support the PMs.

I think we should switch the order of this information - we tell them transcripts scroll automatically, but then we say "but only if  it's an .srt transcript." How about this?

We strongly recommend *(require?)* that you use the SubRip Text (.srt) format for your transcripts. A transcript in the .srt format automatically scrolls next to its associated video as the video plays. A student can click a word in the transcript to jump to the point in the video where that word is spoken. You can include an additional transcript in text (.txt) format. These transcripts may not *(don't?)* scroll automatically.