Moved gallery shortcode styles from _post.scss to _markdown.scss and deleted the unused _post.scss file. Updated import paths in content.scss and global.scss to reflect the new location of markdown styles.

Use `position: absolute` for `<figcaption>`, not affecting the container's width. Also rewrite `<figure>` `<figcaption>` style.

add utility classes ("center" and "full-width")

- centered image
- rounded image
- hide redundant `<p>` tag unexpectedly rendered in `{{< gallery >}}`
- prettier SCSS code

The original method of adding an "external" icon is to append an `<img>` tag to `<a>` tags, but if the reader is viewing the content from a reader where the CSS style does not work, the image is displayed in an unwanted way.

The workaround is to remove the `<img>` tag and implement this feature with only CSS pseudo-element.