vscode: Support opening local documentation if available

Displaying local instead of web docs can have many benefits:
- the web version may have different features enabled than locally selected
- the standard library may be a different version than is available online
- the user may not be online and therefore cannot access the web documentation
- the documentation may not be available online at all, for example because it
  is for a new feature in a library the user is currently developing

If the documentation is not available locally, the extension still falls back to
the web version.

editors/code/src/client.ts

@@ -389,6 +389,7 @@ class ExperimentalFeatures implements lc.StaticFeature {
             serverStatusNotification: true,
             colorDiagnosticOutput: true,
             openServerLogs: true,
+            localDocs: true,
             commands: {
                 commands: [
                     "rust-analyzer.runSingle",