docs: Update PR template.
docs: Update PR template.

- Puts the description info in a comment.
- Adds comments to screenshots section.
- Removes the comments telling people to remove checkbox lines.
- Adds new checklist items for CLA/build/test.

Change-Id: I2d3b66c9b1a16eb489f8d65526a17c4ad4e4fae9

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player