v1.5.3

Changes in this release:

• Corrected AMP CSS to use latest PT Serif font.
• Added brand and voice links in footer.
• Added Prism Markdown syntax highlighting.
• Added files lists option.
• Added reference marks lists option.
• Added example texts option.
• Added heading on Markdown footnotes region.
• Build: replace sass-lint (discontinued) with stylelint.
• Build: remove css-mqpacker (discontinued).