doc: add WIP nvim example config

examples/nvim.md

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+# Neovim configuration
+
+## Register the language server with neovim
+
+TODO: contribute some defaults to the nvim-lspconfig repo and link to that instead.
+
+The following code will register the language server with Neovim, and naively attach it to every buffer. `lsp-ai` is
+configured with the `llama_cpp` backend to run the CodeGemma v1.1 model.
+
+Note: the model configuration is provided as an example/starting point only and I do not vouch for the quality of the
+generations. Adjust to taste.
+
+```lua
+local lsp_ai_config = {
+  -- Uncomment the following line if you use nvim-cmp with the cmp_nvim_lsp source.
+  -- capabilities = require('cmp_nvim_lsp').default_capabilities(),
+  cmd = { 'lsp-ai' },
+  cmd_env = {
+    -- Add required environment variables here, e.g. for CUDA device selection.
+    -- CUDA_VISIBLE_DEVICES = "1"
+  },
+  -- filetypes = { 'python' },
+  root_dir = vim.loop.cwd(),
+  init_options = {
+    -- cmp-ai configuration goes here.
+    memory = {
+      file_store = {}
+    },
+    models = {
+      model1 = {
+        type = "llama_cpp",
+        repository = "mmnga/codegemma-1.1-2b-gguf",
+        name = "codegemma-1.1-2b-Q8_0.gguf",
+        n_ctx = 2048,
+        n_gpu_layers = 999
+      }
+    },
+    completion = {
+      model = "model1",
+      parameters = {
+        fim = {
+          start = "<|fim_prefix|>",
+          middle = "<|fim_suffix|>",
+          ["end"] = "<|fim_middle|>"
+        },
+        max_context = 2000,
+        max_new_tokens = 32
+      }
+    }
+  }
+}
+
+local function attach_buffer()
+  vim.lsp.start(lsp_ai_config)
+end
+
+vim.api.nvim_create_autocmd({"BufEnter", "BufWinEnter"}, {
+  callback = attach_buffer
+})
+```
+
+## Example ghost-text setup
+
+For a copilot-like ghost-text experience, here is an example configuration using the
+[nvim-cmp](https://github.com/hrsh7th/nvim-cmp) plugin, assuming you use the
+[cmp-nvim-lsp](https://github.com/hrsh7th/cmp-nvim-lsp) source. This is **not a full configuration**, please refer to
+the nvim-cmp documentation for a full starter config without ghost text if you need one.
+
+This configuration enables ghost-text in nvim-cmp, and registers a custom comparator that puts `cmp-ai` suggestions
+at the top so that they're the ones being drawn with ghost text.
+
+```lua
+local function ai_top_comparator(entry1, entry2)
+  local comp_item = entry1:get_completion_item()
+  if comp_item ~= nil then
+    if string.sub(comp_item.label, 1, 4) == "ai -" then
+      return true
+    end
+  end
+  comp_item = entry2:get_completion_item()
+  if comp_item ~= nil then
+    if string.sub(comp_item.label, 1, 4) == "ai -" then
+      return false
+    end
+  end
+  return nil
+end
+
+local default_sorting = require('cmp.config.default')().sorting
+local my_sorting = vim.tbl_extend("force", {}, default_sorting)
+table.insert(my_sorting.comparators, 1, ai_top_comparator)
+
+cmp.setup({
+  -- <your nvim-cmp config here>
+  -- ...
+  experimental = {
+    ghost_text = true
+  },
+  sorting = my_sorting
+})
+```
+
+Notes and known issues:
+
+* You'll need a very recent version of `nvim-cmp` for multiline ghost text to work. Note that ghost-text is an
+  experimental feature of `nvim-cmp`.
+* The completions window is currently drawn below the cursor, which hides ghost-text on the following lines. This is
+  a known limitation of nvim-cmp, currently being addressed in PR 1955, so you may want to use the PR 1955 branch for
+  now.
+* The first character of the suggestion is not being properly drawn.