feat: add the img-grid shortcode

[razonyang]

Related to #100 and #103.

{{< bs/img-grid "img-grid-examples" >}}

img-grid-examples is the data filename.

data/img-grid-examples.yaml as follows:

- src: foo.png
  title: Foo
  url: https://example.org/

- src: bar.png
  title: Bar

• src: required, the source/name of image.
• title: required, the title of image, which used as alt and caption.
• url: optional.