[to be scoped] Support more flexible landing page layout

[bmorelli25]

Summary

We might need the ability to support a more customizable landing page layout. For example, the asciidoc system supported full HTML customization, allowing us to do things like:

• Kibana
• Observability
• Elasticsearch

We've heard from our users that they miss pages like these. @georgewallace is mocking something up for ES in elastic/elasticsearch#129589, but we'll need more flexibility with the layout. We definitely don't want to support custom HTML/CSS, but supporting a grid system of some kind would make the content more approachable.