fix(bugfix/fix-footer): update footer

[HoangLiarPhuongLe]

• Delete padding left and padding right of class .footer-section in file footer.css
• Setup width and margin for class .footer-container in file footer.css
• Merge two class .footer-item-top and .footer-item-bottom become class.footer-item for avoid duplicate code
• Rename class.rule become class .footer-rule
• Rounding number the value of opacity
• Split and add a comment for each part as font, color,... of :root{ }
• Add text-transform: uppercase for class navbar-item
• Delete a:hover { }
• Support clickable email and tel number