feat(split-button): add split button

[santiagoballadares]

Add new component

• Change button docs from markdown template to html template
• Add KtSplitButton to button docs

[Isokaeder]

For the secondary style, it is strange to me that the divider line between the buttons disappears when hovering the primary button. However, when hovering the secondary button, the primary line persists

Just a tad inconsistent, if you ask me. If design wants that, so be it. But perhaps clarify with them.

[santiagoballadares]

For the secondary style, it is strange to me that the divider line between the buttons disappears when hovering the primary button. However, when hovering the secondary button, the primary line persists

Just a tad inconsistent, if you ask me. If design wants that, so be it. But perhaps clarify with them.

@Isokaeder Design confirmed that this is the desired behavior.