lsp: Retrieve links to documentation for the given symbol (#19233)

Closes #18924 

Release Notes:

- Added an `editor:OpenDocs` action to open links to documentation via
rust-analyzer

crates/proto/src/proto.rs

@@ -314,6 +314,8 @@ messages!(
     (UsersResponse, Foreground),
     (LspExtExpandMacro, Background),
     (LspExtExpandMacroResponse, Background),
+    (LspExtOpenDocs, Background),
+    (LspExtOpenDocsResponse, Background),
     (SetRoomParticipantRole, Foreground),
     (BlameBuffer, Foreground),
     (BlameBufferResponse, Foreground),
@@ -464,6 +466,7 @@ request_messages!(
     (UpdateProject, Ack),
     (UpdateWorktree, Ack),
     (LspExtExpandMacro, LspExtExpandMacroResponse),
+    (LspExtOpenDocs, LspExtOpenDocsResponse),
     (SetRoomParticipantRole, Ack),
     (BlameBuffer, BlameBufferResponse),
     (RejoinRemoteProjects, RejoinRemoteProjectsResponse),
@@ -552,6 +555,7 @@ entity_messages!(
     UpdateWorktree,
     UpdateWorktreeSettings,
     LspExtExpandMacro,
+    LspExtOpenDocs,
     AdvertiseContexts,
     OpenContext,
     CreateContext,