theme: add estonia

[mpoquet]

Hi and thanks for the amazing work on this package!

This MR adds a new theme that fits most of my use cases.

• Uses a 4-part layout:
  • progress bar (top) (optional)
  • title bar (between progress bar and content) (optional)
  • content (surrounded by margins)
  • bottom bar (optional)
• Defines a highly customizable slide function
• Defines default functions to generate the various bars (user can also use their own function)

I wrote a showcase and customization guide as slides that use the theme: estonia-guide.pdf estonia-guide.typ.txt

I also wrote a bit of documentation for the theme in the book part of this repo but I have mixed feelings about showing all these slides as an example for the theme... I have NOT generated a png version of these slides, I have NOT updated the Pluto notebook that seems to generate previews, and I have NOT generated the book to test whether I introduced some mistakes in it.

[andreasKroepelin]

That looks really cool! Having wonderful childhood memories from Estonia, that country and its colours have a special place in my heart :)

I haven't looked at the code yet but I assume it is quite a bit. Since I just finished a new release of Polylux, I am unsure when I will find the opportunity to spend much time on it again. So please don't be discouraged if it takes a while until this beautiful theme gets merged!