doc: describe palette and highlight semantics

[antoineco]

Warning Ongoing effort, open to early feedback.

Description

Adds a dedicated documentation page for presenting the color palette and explaining the semantics of each color.

The final document will also include a note about higher level design decisions, such as the usage (or not) of italics and the conscious differences between "classic" Vim and Treesitter highlights.

Screenshots

See live preview at https://github.com/antoineco/sainnhe-everforest/blob/palette/palette.md