Add antora docs folder and symlink to spring [DEX-294] (#698)

This will allow to include code sample snippets from the code sample
repository.

If you need to add a different sample add a symlink to a directory in
the root into the docs/modules/ROOT/examples/ directory. E.g.

```
cd docs/modules/ROOT/examples/
ln -s ../../../../spring spring
```

Author: frant-hartm

docs/antora.yml

@@ -0,0 +1,2 @@
+name: hazelcast
+version: 6.0-snapshot

docs/modules/ROOT/examples/spring

@@ -0,0 +1 @@
+../../../../spring