Docs: Add rig instructions for R pre-release testing

[7se7en72025]

Incorporates suggestion from Issue #239 to document the use of 'rig add next' and 'R-next' for installing and switching to the development version of R.

Summary

Adds a new section to the Testing Pre-release R Versions chapter (testing_pre_release_r_versions.qmd) detailing the user-friendly commands for managing R's development version using the rig tool.

• Added a section for rig add next, R-next, and rig default R-next.
• Ensured commands are correctly formatted in bash code blocks.

Fixes #239

Checklist for adding a new chapter or chapters

What should a reviewer concentrate their feedback on?

• Check the placement and clarity of the new section in the documentation flow.
• Everything looks ok?

Acknowledging contributors

List any contributions that need adding to the table of contributors. Common contributions are content (adding to the guide), doc (e.g. README, wiki), infra (e.g. GitHub action), and maintenance (e.g. fixing links). See the full key to Contribution Types.

• Please add @hturner for idea/content.
• For reviewers: All relevant contributions have been added, including reviewers, mentors, contributors to the ideas/planning. See how to acknowledge contributors.