Refine Tailwind CSS customisations for site refresh

[oubiwann]

This is a follow-on from:

• Replace components with tailwind #115

Tasks:

• Second draft (styling)
  • reduce the total height of the top-nav by a bit
  • get the logic fixed for currently selected page in the top-nav
  • add coloured buttons to the "Download" drop-down
  • fine-tune tabs on front page carousel
    • shorter height
    • lighter text
    • fix the selected bottom border
    • maybe put a filler border above the content, below the tabs
  • improve the styling for the description text in each tab of the carousel
  • update REPL image with a new one (updated LFE and Erlang versions, better sized, etc.)
  • Liffybot background image in "Buildit" section
  • put some colour in the link buttons at the bottom of the book cards
  • put some colour in the link buttons at the bottom of the video cards
  • fix up discord link in final callout
  • add more vertical padding around final callout
  • add background colour for light theme in final callout
  • add missing links to footer
  • ensure all rounded corners use the same radius
  • reduce the size of copyright text
  • add greyscale LFE mug icon to footer

View deploys of the progress here:

• https://staging.lfe.io/

Update: site refresh has been published:

• https://lfe.io

Resources:

• https://tailwindcss.com/docs/
• https://tailwindui.com/
• https://www.preline.co/docs/index.html
• https://www.preline.co/examples.html

[oubiwann]

This ticket will continue the work of the epic (linked about) in the following branch:

• https://github.com/lfe/lfe.github.io/tree/tailwind-migration

[oubiwann]

Phew! Finally finished this one up!