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/lsp/src/lsp.rs

@@ -762,6 +762,7 @@ impl LanguageServer {
                 }),
                 experimental: Some(json!({
                     "serverStatusNotification": true,
+                    "localDocs": true,
                 })),
                 window: Some(WindowClientCapabilities {
                     work_done_progress: Some(true),