Best practices: add recommended presence (#386)

* Add recommended presence

* feed_info.txt best practices

* route_short_name best practice

* agency_id best practice

* timepoint best practice

* inlining image for shape_dist_traveled

* shape_dist_traveled field best practices

* block_id field presence best practice

* Clearer recommended definition

* clarity improvements

* Remove "may" for block_id in frequency based trips

* fix shape_dist_traveled cell formatting