1039 Update get_started notebook to markdown doc (#1045)

Fixes #1039 .

### Description
This PR updated the `get_started` notebook of bundle to be a markdown
doc as it can't execute.

### Checks
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [ ] Notebook runs automatically `./runner [-p <regex_pattern>]`

Signed-off-by: Nic Ma <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Nic-Ma

bundle/README.md

@@ -1,5 +1,5 @@
 # MONAI bundle
-This folder contains the getting started tutorial and code examples of training / inference for MONAI bundle parser.
+This folder contains the `getting started` tutorial and below code examples of training / inference for MONAI bundle parser.
 
 ### [introducing_config](./introducing_config)
 A simple example to introduce the MONAI bundle config and parsing.