bookdown header for old version of docs should have links to latest.

[mitzimorris]

Summary:

We need to add a header to the versioned docs so that old versions have links to latest.

Description:

Currently, the docs site is set up to serve a static site via GitHub which contains the Stan User's Guide, Functions Reference, and Reference Manual docs both in bookdown and pdf formats. GitHub serves from the Doc's repo directory named "docs", and the resulting URLs look like:

https://mc-stan.org/docs/2_18/stan-users-guide/index.html
...
https://mc-stan.org/docs/2_21/stan-users-guide/index.html

the following URL will redirect to the latest docset:

https://mc-stan.org/docs/stan-users-guide/index.html

We need to change the bookdown html so that old versions have a div running across the top of the page with this link, which will always redirect to the latest version.

Additional Information:

Need someone who understands bookdown, hugo, and is good with css, etc.
See https://bookdown.org/yihui/blogdown/custom-layouts.html for more info.

Current Version:

v2.18.0

[mitzimorris]

screenshot of what can be done with bookdown::gitbook package:

circles indicate links to unversioned file, which is a redirect to latest.

[mitzimorris]

#113