Document Jsonnet best practices

[simu]

As more people start writing Commodore components, questions regarding Jsonnet best practices are popping up more frequently.

While we already have #36 to document component architecture and interface principles and #40 to establish a Jsonnet style convention and guide, other best practice areas also need documentation, e.g.:

• best practices when using kube-libsonnet.

• Best practices for internal component structure, such as what belongs in a component library, when does it make sense to have multiple jsonnet files in component/, etc.

• [x] Merging External Config Data

• [ ] Loops and Filters

• [ ] Component Library Functions

• [ ] Conditional Keys and Files

• [ ] Using kube-libsonnet

• [ ] Using Library functions

• [ ] CRD Group Versions

• [ ] Container Image & Helm Chart Versions

• [ ] Multiline Strings

• [ ] Component Structure

• [x] Randomize (cron) schedules -> see #96

[srueg]

Draft PR with discussion notes: https://github.com/projectsyn/documentation/pull/86