docs: add shortcodes and CSS

[JTorreG]

Proposed changes

Adds the call-out and before-you-being used in the docs repo to the theme.

Adds the CSS that was used in custom tables for NGINX One to the default styles for all tables.

before:

after:

Follow ups after merge and release:

• Remove the shortcodes from the layouts folder in the docs repo
• Remove the custom styles in the NGINX one docs/branch
• Bump the theme/rebuild the docs in the docs repo

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[mjang]

While I'm OK and 👍 this, I'd like to see @LukeBillington 's approval before merge.