Minor HTML semantics improvments

[ggrochow]

Fixes some minor semantics issues found with axe devtools

• include lang=en-US on homepage tag
  • fixes - elements must have a lang attribute
• ensure h1 Diesel Logo link has a "discernible text" with aria-label
  • fixes - Links must have discernible text & Headings should not be empty
• wrap main page content in
  tag

  • fixes - Document should have one main landmark & All page content should be contained by landmarks