Add hint boxes and codetabs to landing-page

[samredai]

This adds some shortcodes to the landing-page theme that allows us to include hint-boxes and codetabs (code blocks in a tabbed container for multiple languages). Here's an example of what this looks like:

Here's the code directly in the markdown file that utilized the shortcodes to create what's seen above.

---
url: example
toc: true
---
# Example

## Hint Boxes

{{< hint info >}}
This is an info hint box
{{< /hint >}}

{{< hint success >}}
This is a success hint box
{{< /hint >}}

{{< hint warning >}}
This is a warning hint box
{{< /hint >}}

{{< hint error >}}
This is an error hint box
{{< /hint >}}

## Codetabs

{{% codetabs "Example" %}}

{{% addtab "Python" checked %}}
{{% addtab "Java" %}}
{{% addtab "Scala" %}}

{{% tabcontent "Python" %}}
```py
print("Hello, World")
```
{{% /tabcontent %}}

{{% tabcontent "Java" %}}
```java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World");
    }
}
```
{{% /tabcontent %}}

{{% tabcontent "Scala" %}}
```scala
object HelloWorld extends App {
    println("Hello, World")
}
```
{{% /tabcontent %}}

{{% /codetabs %}}

The reason for adding this is to use them for the quickstarts which will be a part of the landing-page.

[rdblue]

Looks great. Thanks, @samredai!