Rename Max Mode to Burn Mode throughout code and docs (#31668)

Follow up to https://github.com/zed-industries/zed/pull/31470.

I started looking at config and changed preferred_completion_mode to
burn to only find its max so made changes to align it better with
rebrand. As this is in preview build now.

This doesn't touch zed_llm_client. Only the Zed changes the code and doc
to match the new UI of burn mode. There are still more things to be
renamed, though.

Release Notes:

- N/A

---------

Signed-off-by: Umesh Yadav <git@umesh.dev>
Co-authored-by: Danilo Leal <daniloleal09@gmail.com>

crates/agent_settings/src/agent_settings.rs

@@ -689,14 +689,14 @@ pub struct AgentSettingsContentV2 {
 pub enum CompletionMode {
     #[default]
     Normal,
-    Max,
+    Burn,
 }
 
 impl From<CompletionMode> for zed_llm_client::CompletionMode {
     fn from(value: CompletionMode) -> Self {
         match value {
             CompletionMode::Normal => zed_llm_client::CompletionMode::Normal,
-            CompletionMode::Max => zed_llm_client::CompletionMode::Max,
+            CompletionMode::Burn => zed_llm_client::CompletionMode::Max,
         }
     }
 }