Adding some formatting instructions for contributing

[AoifeHughes]

Addresses: #620

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[Copilot]

Pull Request Overview

This PR adds a new "Code Formatting" section to the contributing guide to ensure consistent styling across the Turing codebase.

• Introduces instructions for installing JuliaFormatter v1 in the global environment.
• Provides a command to run JuliaFormatter on all project files before committing.

Comments suppressed due to low confidence (2)

developers/contributing/index.qmd:61

• The Pkg.activate() call without arguments may activate the local project instead of the global environment. Consider specifying the global environment path explicitly (e.g., Pkg.activate("~/.julia/environments/v1.x")) or omitting it, since Pkg.add defaults to the active user environment.

julia -e 'using Pkg; Pkg.activate(); Pkg.add(name="JuliaFormatter", version="1"); Pkg.pin("JuliaFormatter")'

developers/contributing/index.qmd:52

• [nitpick] It may be helpful to mention that users can customize formatting rules with a .JuliaFormatter.toml configuration file in the repo root to enforce project-specific style options.

### Code Formatting

[penelopeysm]

Nice! Just a couple of tiny suggested changes.

General request: could you insert semantic line-breaks, please? Just to keep diffs easier in the future.