Update links for configuring c toolchain

[jamesmbaazam]

Description

This PR closes #637.

Previously, we were linking a page for installing RStan instead of RTools. In this PR, I have linked to guides for each operating system.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
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After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.