Add --with-codestart create-extension option to tutorial

[joes]

Description

Document the option -DwithCodestart in the tutorial for generating new Codestart extensions.

Background
The option -DwithCodestart was provided in #26628 by @ia3andy but I did not see any documentation for it.

The tutorial suggests that the easiest way to create a codestart extension is by starting with a Quarkus project. However, with the addition of the -DwithCodestart option this is no longer the easiest way (imho) so I changed the wording in the tutorial wrt this to make it more neutral.

[joes]

Removed unneeded comment placeholders in the usage example

[gastaldi]

That looks good to me. It would be nice to also document using the Quarkus CLI (which we highly recommend):

$ quarkus create extension greeting --codestart                 ✔
Looking for the newly published extensions in registry.quarkus.io

Detected layout type is 'quarkiverse'
Generated runtime artifactId is 'quarkus-greeting'


applying codestarts...
📚  java
🔨  maven
📦  quarkus-extension
🔧  git
🚀  devmode-test
🚀  extension-base
🚀  extension-codestart
🚀  integration-tests
🚀  quarkiverse
🚀  unit-test

-----------
 👍  extension has been successfully generated in:
--> /private/tmp/quarkus-greeting
-----------
Navigate into this directory and get started: quarkus build

[ia3andy]

Thanks @gastaldi for the review :)

[joes]

I documented Quarkus CLI as suggested by you @gastaldi.

[gastaldi]

LGTM

[gsmet]

@ia3andy could you have a look at this one?

[ia3andy]

LGTM

[joes]

Thank you for all the input @gastaldi!