Improve Crosswalk page introduction and contribution instructions

[meldra]

Also tweak the list partial to make the list easier to read
and the pagination easier to find/use. One crosswalk page
may need its title fixed now that the logo isn't enormous.

[progval]

and the pagination easier to find/use

IMO we can remove it entirely. We don't have thousands of crosswalks, and having them all on the same page makes them easier to find.