Some API documentation changes

[leeper]

This is a documentation-only change that:

• notes the R client for dataverse is now available from R's central repository, CRAN and updates a link to it from the SWORD API page
• standardizes some of the formatting of example across the different API guide pages
• moves language describing the API key to the main API index page (from the native API page)

[coveralls]

Coverage remained the same at 10.038% when pulling f5c5a4f on leeper:master into b0fd002 on IQSS:master.

[coveralls]

Coverage remained the same at 10.038% when pulling 2b01a60 on leeper:master into b0fd002 on IQSS:master.

[pdurbin]

@leeper thanks for the pull request! I just advanced it to the "Code Review" column at https://waffle.io/IQSS/dataverse

[pdurbin]

@leeper if you happen to know if it's easy to change the destination branch from master to develop please let me know.

[leeper]

@pdurbin Did that work?

[pdurbin]

@leeper much better. Thanks!

[pdurbin]

@leeper I'm thinking I'll merge these changes with another API Guide improvement effort for #3192.

[leeper]

Sounds great!

[pdurbin]

@leeper I'm closing this issue in favor of pull request #3920

The only change I backed out of is the curl commands in the SWORD docs. We need to figure out a sane way wrap long lines, as @dlmurphy and I have discussed in #3577.