Smal web page cleanup

Hi webtiming folks.

I did a little cleanup of some of the pages at
https://www.w3.org/community/webtiming/ to have them better reflect
developments over the last couple of months.

Also, the Sequencer documentation has been extended with some high-level
considerations around sequencing and the implied programming model. This
should be of particular interest for activities related to timed text,
 timed transcripts, timed animations etc.

http://webtiming.github.io/sequencer/overview.html

Ingar