pages/cfp.md: add some changes drafted by the content team

Also uncomment the whole content.

The timeline is still incomplete:

 * Early submission deadline: TBD
 * Acceptance notification: TBD
 * Final submission deadline: TBD

@santiago @gwolf @tassia @lenharo @mbanck we need to define the above.

This bit I updated base on !10 (merged) :

-* If you need to apply for a bursary, submit your proposal by May 8th.
+* If you need to apply for a bursary, submit your proposal by May 7th.

The academic track content I did not add to this MR. Feedback is welcomed :)