Deduplicate pages in CmdStan guide

Submission Checklist

[x] Builds locally
[x] New functions marked with <<{ since VERSION }>>
[x] Declare copyright holder and open-source license: see below

Summary

The cmdstan guide has previously had an "Intro" page and a "Config" page for each algorithm. Over time these have grown to have mostly the same information as each other, with the downside that not both have been updated as things have evolved. This PR does some pretty direct cut-and-paste followed by deduplication to give us one page on each.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
Documentation: CC BY-ND 4.0 (https://creativecommons.org/licenses/by-nd/4.0/)

stan-dev / docs