Linting Updates (Removed Vale, Added Prettier, MarkdownLinter, ESLint)

[sifex]

Context

This updates Sigma docs with 3x linting & formatting programs to help us reach a more opinionated clean state for Sigma docs.

• Adds linting as a required step in .github/workflows
• Adds the following linters / formatters and their reason
  • ESLint – to handle JS / Vue code best-practices
  • MarkdownLinter CLI v2 – to handle correct structure of Markdown documents
  • Prettier – mostly to handle tailwind class ordering and HTML / Markdown formatting
• Removes Vale as it wasn't really used.