3.x example removal

[barchetta]

Description

Example removal. Helidon 3 examples now live in the helidon-examples repository

This PR:

1. Deletes examples from Helidon 3
2. Updates the examples top level README to direct users to helidon-examples
3. Updates the examples profile to include a module called helidon-examples (instead of examples)
4. Adds a build-examples script that clones the helidon-examples repository into the helidon-examples directory (if it doesn't already exist) and builds the examples.
5. Adds helidon-examples to .gitignore

The last three items establishes a pattern that can be used by developers to more easily work on changes to examples and core helidon concurrently.

Documentation

Any documentation that references examples will need to have those links updated.

Developing Examples

To make it a bit easier working in two different repositories (helidon and helidon-examples) you can use this workflow:

1. Checkout your 3.x branch and build it
2. Run etc/scripts/build-examples.sh. This will clone the examples repository's dev-3.x branch into helidon-examples and build it
3. In your IDE under Maven configuration select the examples profile.

If you need to coordinate a (possibly breaking) change in the main helidon repo and in helidon-examples

1. Create your branch in both local repositories
2. Create pull requests against both repositories.
3. The PR against helidon might fail the examples validation. But it will not block the PR.
4. The PR against helidon-examples might fail validation. If so it will block the PR.
5. Merge the PR against helidon
6. Re-trigger validation on the helidon-examples PR which should now pass. Then merge.