docs: update recommendation on installing themes

Instead of themes, flavors are now recommended. They include both the
theme and the code highlighting theme, so it's a more complete package.

They are also easier to keep up to date, as they don't require
overriding all of the theme's settings in the user's configuration.

> The purpose of separating them is to allow users to customize their
> preferences more conveniently on top of an existing flavor, without
> having to modify those flavor files. This makes it easier to update, as
> there won't be conflicts when pulling from Git.
>
> https://yazi-rs.github.io/docs/flavors/overview/

README.md

@@ -317,12 +317,9 @@ I like.
 - <https://github.com/Rolv-Apneseth/starship.yazi> is a port of the
   [starship prompt](https://starship.rs) to yazi. It allows reusing the prompt
   you are using in your shell in yazi.
-- <https://github.com/catppuccin/yazi> ports the catppuccin theme to yazi.
-- <https://github.com/catppuccin/bat> can be used to change the syntax
-  highlighting theme yazi uses to preview files. See
-  [this discussion](https://github.com/sxyazi/yazi/discussions/818) or
-  [my config](https://github.com/mikavilpas/dotfiles/commit/bb07515f69d219fd3435d222fcb2d80d27a25025#diff-973b37f40e024ca0f7e62f2569efce24ad550d0352adc8449168ac950af9eaf5R8)
-  for an example of using it
+- <https://github.com/yazi-rs/flavors> houses many of the most popular themes as
+  yazi flavors, check it out! They typically also include matching code
+  highlighting.
 
 ## Contributing