Build book-theme variant with landing page support

[agoose77]

In #5339, I explored a potential direction for a landing page feature in MyST. As a part of this work, I identified some useful component types and started building a demo site for CryoCloud that can be rendered by this custom theme.

### Tasks
- [x] Build custom renderers for image-split CTA
- [x] Build custom renderers for justified CTA
- [x] Build custom renderers for centered CTA
- [x] Build custom renderers for logo cloud
- [ ] https://github.com/jupyter-book/myst-theme/pull/536
- [ ] https://github.com/jupyter-book/myst-theme/pull/535

PR

jupyter-book/myst-theme#531

Related Issues

• Frontmatter block does not use grid layout in book-theme jupyter-book/myst-theme#526
• Minimal required features for the Project Pythia landing page projectpythia-mystmd/projectpythia-mystmd.github.io#1

Follow up work

Leverage this new theme for JB 2.0

Definition of Done

• A landing-page aware book theme has been deployed as a static site alongside demonstration content.
• The documentation for the this custom theme is deployed alongside it.

[agoose77]

I think this unit of work is "done". Next, we'll review what works and what doesn't, and pull in some stakeholders.