Revamp the Lookups page

[vy]

As agreed in the site structure document, revamp the "Development" page such that

• Outline:
  • What is a lookup?
    • Also explain the interpolation difference between configuration and runtime
    • List lookups + link to plugin reference pages
  • Extending lookups
• Target 2.x, port to main

Interpolation rules

Interpolation rules of lookups in different places are very complicated. Consider following cases:

• ${foo} passed to <JsonTemplateLayout in log4j2.xml
• $${foo} passed to <JsonTemplateLayout in log4j2.xml
• ${foo} passed to <JsonTemplateLayout in an event template configuration file (e.g., FooLayout.json)
• $${foo} passed to <JsonTemplateLayout in an event template configuration file (e.g., FooLayout.json)
• ${foo} passed to <EventTemplateAdditionalField of <JsonTemplateLayout in log4j2.xml
• $${foo} passed to <EventTemplateAdditionalField of <JsonTemplateLayout in log4j2.xml
• ${foo} passed to JsonTemplateLayout.Builder in an event template configuration file programmatically
• $${foo} passed to JsonTemplateLayout.Builder in an event template configuration file programmatically

We should provide assistance to guide users in these cases, document, and verify(!) them.

Lookups vs property substitution

In several places of the document, lookups and property substitution are used interchangeably, even though they are not. We should clarify this ambiguity, in particular, in layouts.