Revamp the Extending, Plugins, Architecture, and Programmatic configuration pages

[vy]

This incomplete work ports #2696 to main.
The following tasks still need to be done:

• Adapt plugins.adoc for Log4j 3
  • Artifact has changed
  • Package paths have changed (hence the Javadoc links)
  • There are new annotations: @Namespace, @Category, @PluginOrder, @Ordered, etc.
  • Some annotations are removed: @Order
  • There are no connections between plugins.adoc and dependencyinjection.adoc
• Revamp dependencyinjection.adoc
  • No Javadoc or source code links
  • Sectioning can be improved to guide the user better
  • Connection with the plugins.adoc can be improved
• Fix plugin-preliminaries.adoc
• Fix plugin-related parts (some still refer to category, etc.) of
  • appenders.adoc
  • configuration.adoc
  • extending.adoc
  • filters.adoc
  • layouts.adoc
  • lookups.adoc
  • pattern-layout.adoc

Once #2696 is ported, #2692 and #2715 needs to be ported (in given order) too.

[ppkarwasz]

Sorry, I approved the wrong PR. 😉

I'll add a review later.

[jvz]

Looks good so far! Now I'll look through your TODO list.

[jvz]

The PlantUML docs have some other relation types you can use here like o-- for aggregation and *-- for composition, though I've used the aggregation relation before to denote when one class has an array or collection of another class. What you've done here, though, is probably easier to understand for people who don't write UML diagrams :)

[jvz]

This UML diagram is pretty awesome!

[jvz]

Could be worth mentioning that Logger and LoggerConfig work via a bridge pattern where Logger provides the API and LoggerConfig provides the implementation. This is one key architectural constraint that allows for reconfiguration without dropping any log events being processed at the time of reconfiguration. It's also one of the reasons why many methods of manual configuration of appenders and such is so hard to do properly and why we offer various utility methods and configuration DSLs to sort of hide away the lifecycle handling, concurrency, etc.

[jvz]

So far, I've mostly updated the plugins page, but I still have some things to add about order annotations, probably some info about conditional annotations, and a link to the DI page.

[jvz]

Remaining tasks:

• Update existing configuration factory plugin classes to use @Ordered instead of @Order.
• Would be nice if there were a way to still support @Order, though it's in log4j-core, not in log4j-plugins.
• plugins.adoc should have a section about conditional annotations nearby the validation annotations.
• plugins.adoc should include info about plugin aliases (which are used for plugin names and plugin attributes).
• The DI page could still use some structuring for sections.