docs: add plugin manager installation examples

[Arvuno]

Summary

Add plugin manager installation examples (vim-plug, dein.nvim, lazy.nvim, packer.nvim) to the Installing Apprentice section.

Problem

The README currently only shows manual installation (Pathogen example, :help packages example, and AUR). Users of modern plugin managers like vim-plug, dein.nvim, lazy.nvim, and packer.nvim have no documented way to install Apprentice via their preferred tool.

Solution

Add a new "Plugin managers" subsection with code examples for:

• vim-plug
• dein.nvim
• lazy.nvim
• packer.nvim

Each example shows the minimal configuration needed to install Apprentice via that plugin manager.

Files Changed

• README.md — Added plugin manager installation examples section

Testing

• Verified code examples are syntactically correct for each plugin manager
• No functional changes to the colorscheme itself

[romainl]

Hello. I'm OK with adding a vim-plug example because it is the most common third-party plugin manager in Vimland. In fact, the Pathogen example should probably be replaced with a vim-plug one.

But adding examples for various Neovim plugin managers would imply that Neovim is officially supported while it is not.

So yeah, I can accept this PR if you restrict it to that "replace pathogen with vim-plug" scope but I can't accept anything related to Neovim.