chore(ci): Add Vale prose linting for documentation

[jstirnaman]

Summary

Adds Vale prose linting to complement the existing markdownlint setup. While markdownlint checks Markdown formatting, Vale checks prose style, grammar, and terminology consistency.

Changes

• .vale.ini: Configuration using Google Developer Documentation Style Guide
• .vale/styles/Telegraf/: Custom rules for Telegraf documentation:
  • Latin.yml: Flags Latin abbreviations (e.g., i.e., etc.) - suggest plain English
  • Grammar.yml: Flags common grammar issues ("allows to", "in order to", "In case")
  • Terms.yml: Enforces correct product name capitalization (InfluxDB, Telegraf)
• .github/workflows/vale.yml: Runs Vale on PR changes to Markdown files
• docs/developers/PROSE_STYLE.md: Documentation for contributors
• CONTRIBUTING.md: Updated with Vale reference

How it works

• Runs automatically on PRs that modify .md files
• Only lints changed files (not entire codebase)
• Reports issues as PR annotations
• Currently configured as non-blocking (fail_on_error: false)

Running locally

# Install Vale
brew install vale  # macOS

# Download style packages
vale sync

# Lint files
vale plugins/inputs/cpu/README.md

Test plan

• Vale configuration validates (vale ls-config)
• Custom rules are syntactically correct
• Workflow file is valid YAML
• Documentation is complete and accurate