Add missing rmarkdown vignette dependency for linux R-devel checks

[Copilot]

The linux (R-devel) GitHub Actions job was failing during vignette rebuilds because the package metadata did not declare rmarkdown, even though the package ships multiple *.Rmd vignettes. This change aligns DESCRIPTION with the actual vignette toolchain so CI installs the required package.

• Root cause

  • Vignette rebuilds in CI failed with there is no package called 'rmarkdown'
  • The package includes R Markdown vignettes under vignettes/, but DESCRIPTION only listed knitr in Suggests

• Change

  • Add rmarkdown to Suggests in DESCRIPTION
  • Leave the existing VignetteBuilder: knitr configuration unchanged

• Effect

  • Ensures environments performing package checks and vignette builds install the full R Markdown dependency set
  • Removes the metadata gap that caused linux (R-devel) to fail

Suggests:
    forecast,
    viridis,
-   knitr
+   knitr,
+   rmarkdown

[Copilot]

Copilot review overview

🟢 Approval recommended

The metadata change addresses the missing CI dependency without introducing unresolved issues.

Review effort: Lite
Findings: None

What changed in this PR

Adds the missing rmarkdown dependency for R Markdown vignette builds and R-devel CI checks.

Changes:

• Adds rmarkdown to Suggests.
• Preserves VignetteBuilder: knitr.

File	Description
DESCRIPTION	Declares the required vignette dependency.

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