feat: Add separate Slide Layouts documentation.

[ZimChristine]

Description

This PR depends on https://github.com/FormidableLabs/spectacle/pull/1221

Fixes # (issue)

Type of Change

• [ X ] This change requires a documentation update

How Has This Been Tested?

The markdown in slide-layouts.md was tested in the VSCode Preview pane but has not been verified that it works in a Docusaurus-manner.

Checklist: (Feel free to delete this section upon completion)

• [ x ] I have included a changeset if this change will require a version change to one of the packages.
• [ X ] I have performed a self-review of my own code
• [ X ] I have commented my code, particularly in hard-to-understand areas
• [ X ] I have made corresponding changes to the documentation
• [ x ] I have run pnpm run check:ci and all checks pass
• [ na ] I have added tests that prove my fix is effective or that my feature works
• [ x ] My changes generate no new warnings
• [ TODO ] Any dependent changes have been merged and published in downstream modules