docs: add missing boards to reference home page

Author: machow

docs/api/index.rst

@@ -32,12 +32,18 @@ Boards abstract over different storage backends, making it easy to share data in
 .. list-table::
    :class: table-align-left
 
-   * - :func:`.board_folder`, :func:`.board_local`
+   * - :func:`.board_azure`
+     - Use an Azure storage container as a board
+   * - :func:`.board_folder`, :func:`.board_local`, :func:`.board_temp`
      - Use a local folder as a board
    * - :func:`.board_rsconnect`
      - Use RStudio Connect as a board
    * - :func:`.board_s3`
      - Use an S3 bucket as a board
+   * - :func:`.board_gcs`
+     - Use a Google Cloud Storage bucket as a board.
+   * - :func:`.board_url`
+     - Use a dictionary of URLs as a board
    * - :func:`.board`
      - Generic board constructor