docs: enrich the landing page with basic concepts (#472)

Improve the definition of Pebble and add fundamentals.
Co-authored-by: Ben Hoyt <[email protected]>

docs/index.md

@@ -1,8 +1,12 @@
 # Pebble
 
-_Take control of your internal daemons!_
+**Pebble** is a lightweight Linux service manager.
 
-**Pebble** helps you to orchestrate a set of local service processes as an organised set. It resembles well known tools such as _supervisord_, _runit_, or _s6_, in that it can easily manage non-system processes independently from the system services, but it was designed with unique features that help with more specific use cases.
+It helps you orchestrate a set of local processes as an organised set. It resembles well-known tools such as _supervisord_, _runit_, or _s6_, in that it can easily manage non-system processes independently from the system services. However, it was designed with unique features such as layered configuration and an HTTP API that help with more specific use cases.
+
+If you need a way to manage one or more services in a container, or as a non-root user on a machine, Pebble might be for you. It handles service logs, service dependencies, and allows you to set up ongoing health checks. Plus, it has an "HTTP over unix socket" API for all operations, with simple UID-based access control.
+
+Pebble is useful for developers who are building [Juju charms on Kubernetes](https://juju.is/docs/sdk/from-zero-to-hero-write-your-first-kubernetes-charm), creating [Rocks](https://documentation.ubuntu.com/rockcraft/en/latest/explanation/rocks/) or Docker images, or orchestrating services in the virtual machine.
 
 ## In this documentation