Document formatting expectations and supported Prettier workflow

[Copilot]

The repository had no contributor-facing guidance for its supported formatting workflow, so the expected formatting rules and the intentionally light linting posture were undocumented. This PR adds that guidance and clarifies the repo’s Prettier config and file-type expectations without introducing extra tooling.

Conference/Workshop Details

• Name, Location, Date(s), Website URL and Status are required
• X / Twitter is Optional
• Location should be City + Country or Online
• Status can include information about registration, pricing, calls for proposals and can include links to all the above.
• This PR does not add or modify conference data; it updates contributor documentation for formatting expectations and local workflow guidance.

Checklist

• I have added the conference/workshop to the correct YAML file (_data/current.yml or _data/past.yml)
• The entry includes name, location, date(s), url and accurate status
• The entry is in chronological order (soonest to furtherest away)
• If there are any special characters in the name field (: or ; or '), the name must be in quotes (")
• I have checked for duplicates to avoid listing the same event twice
• Build runs successfully

Additional context

• Added a concise contributor section covering the repo’s existing .prettierrc and .prettierignore settings.
• Documented the supported local formatting commands and the expected formatting behavior for Markdown, YAML, HTML, CSS/SCSS, and JavaScript.
• Clarified that the repository intentionally avoids additional linting or pre-commit tooling unless it adds clear value without creating conflict with the existing validator and Jekyll build flow.

• Fixes Phase 3: Document Formatting Expectations and Linting Tooling #935