Update Windows doc to use RTools44, add ARM64 instructions

[andrjohns]

Submission Checklist

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

Summary

With the R4.0 release, the Rtools44 toolchain has also been release, including a stable ARM64 version. This PR updates the doc to recommend the use of RTools44, and also adds instructions for using the ARM64 toolchain on windows

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):
Andrew Johnson

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/)