docs(gemini): add gemini adapter setup guide

Author: Davidyz

doc/configuration/adapters.md

@@ -483,3 +483,39 @@ require("codecompanion").setup({
 }),
 ```
 
+## Setup: Gemini
+> This is NOT the Gemini-CLI ACP adapter!
+
+The Gemini adapter uses Google's official [openai-compatible](https://ai.google.dev/gemini-api/docs/openai) endpoint, with one difference: the `reasoning_effort` key is replaced by `thinking_budget`, which controls the number of tokens used by the reasoning process.
+
+```lua 
+require("codecompanion").setup({
+  adapters = {
+    http = {
+      gemini = function()
+        return require("codecompanion.adapters").extend("gemini", {
+          schema = {
+            thinking_budget = {
+              default = -1,
+            },
+          },
+        })
+      end,
+    },
+  },
+  strategies = {
+    chat = {
+      adapter = "gemini",
+    },
+    inline = {
+      adapter = "gemini",
+    },
+  },
+}),
+```
+
+The value of `thinking_budget` can be the following:
+
+- `-1`: dynamic thinking (default).
+- `0`: disable reasoning.
+- A valid positive integer (see [Google's documentation](https://ai.google.dev/gemini-api/docs/thinking#set-budget) for supported thinking budgets for each models).