docs: add native Neovim 0.11+ setup guide

[krisajenkins]

Summary

• Restructures the Neovim setup guide into two approaches: native (0.11+) and plugin (0.9+)
• The native approach uses vim.lsp.enable(), vim.filetype.add(), and the lsp/ config directory — no nvim-lspconfig dependency required
• Fixes the plugin setup instructions to handle the monorepo subdirectory correctly (lazy.nvim needs rtp:prepend and ftdetect pointed at packages/nvim-allium)
• Adds a note about Neovim 0.11's default LSP keymaps (grn, gra, grr)

Test plan

• Verify native setup instructions work on a clean Neovim 0.11+ config
• Verify plugin setup instructions work with lazy.nvim
• Verify :TSInstall allium succeeds with the monorepo location field

[henrygarner]

Superseded by the extraction of nvim-allium into its own repository: https://github.com/juxt/nvim-allium

The new repo incorporates everything from this PR:

• Native 0.11+ LSP support via vim.lsp.config()/vim.lsp.enable(), with lspconfig fallback for 0.9+
• Bundled ftdetect/allium.lua for automatic filetype detection
• No monorepo workarounds needed — { "juxt/nvim-allium" } in lazy.nvim just works
• Tree-sitter grammar registered via location field pointing at the allium-tools monorepo

Thanks for the PR, Kris — it was the catalyst for splitting this out properly.

[krisajenkins]

Awesome! That's a huge improvement. Glad I created a bunch of work for you helped. 😁