Update README setup instructions

*Why*:
* The setup steps were stale: they pinned yarn v2 while the repo's package.json declares yarn@3, and they referenced an `install:sdk` step that no longer exists.

*What:*
* Bump `npm install -g yarn@2` to `yarn@3` to match `packageManager` in package.json.
* Drop the obsolete `yarn run install:sdk` step; `@databricks/sdk-experimental` is a regular npm dependency installed by `yarn install`.

*Verification:*
* Confirmed no `install:sdk` script remains in any package.json and that `@databricks/sdk-experimental` is a dependency in packages/databricks-vscode/package.json.
* Confirmed `packageManager` is `yarn@3.2.1`.

Co-authored-by: Isaac

Author: rugpanov

README.md

@@ -21,15 +21,11 @@ Currently, we have the following packages:
 Prepare yarn:
 
 ```
-npm install -g yarn@2
+npm install -g yarn@3
 yarn install
 ```
 
-Prepare Databricks JavaScript SDK:
-
-```
-yarn run install:sdk
-```
+The Databricks JavaScript SDK (`@databricks/sdk-experimental`) is a regular npm dependency and is installed automatically by `yarn install` — no separate step is required.
 
 Prepare Databricks CLI: