Improve extension author guide

[emmanuelbernard]

• Describe where to put APIs (runtime, deployment etc?)
• Describe where Substitutions and templates should live
• Dealing with META-INF/services files Extension author guide: explain how to deal with META-INF/services files #2299
• Offer a getting started section to ease the entry into Extension writing Building my first extension guide #2406

[emmanuelbernard]

See also #2406 for an easier guide start

[cmoulliard]

Here is a new list of things to be improved after spending 2 days to try to create a simple Servlet Extension - https://github.com/cmoulliard/quarkus-my-ext

To Do

• Update the maven archetype to create an extension as it is outdated
• Review Guide in order to avoid to have to create the extension under the quarkus/extensions project
• Add Junit test to show how to test deployment/runtime code
• Explain which BOM o be used and dependencies to include to register servlet, ... on undertow
• Explain when we must use @recorder, @record
• Better introduce the role of the deployment and runtime` module, how the code is build and next used by a Quarkus application
• Do we need the runtime module for all the extensions ? If yes -> Why and when we should use it