Documentation: Improve landing page

[fhennig]

part of: stackabletech/documentation#408

example of an improved landing page: https://docs.stackable.tech/home/stable/druid/index.html

The new landing page should feature:

• meta tags and a meta description
• an introduction of the tool
• a section about getting started
• a description and diagram of the resources it creates
• a description of its dependencies or other operators that interact with it
• a link to demos that use the operator
• the versions (as already present)

The Pod Placement page should be moved into the usage guide