Add better include structure for our conference pages

[ericholscher]

Currently some of our conference pages have includes, and some don't. We should be standardizing the information about each conference event in an include, and then have the per-year conference page have details about the actual conference logistics.

A good example is our Writing Day page: https://github.com/writethedocs/www/blob/master/docs/conf/portland/2020/writing-day.rst#L7 -- it has an include with details, and then conference-specific details after that. We should go through all our pages, and create similar structures.

[plaindocs]

Worth thinking about the in person and online variants here as well.

[rukundob451]

@ericholscher, is this still available to be worked on?