Fixes `cargo doc` warnings and README changes

[desaikd]

Description of changes:

This PR works on fixing cargo doc warnings and README changes.

List of changes:

• modifies README and example with latest API changes
  • Moves the examples folder into ion-schema to correctly trigger examples using cargo run --package ion-schema --example schema ....
  • Changes README example to use latest API changes
• Fixes cargo doc warnings
  • cargo doc generated some warnings for linking to the right crate structures.
  • Removes usage of modules that are crate level in public doc comments.
  • Change scope for all structures of types module to crate level except TypeDefinition which is used to return types from a schema.
• Fixes isl module doc test to construct an ISL type and convert it into a resolved type.

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.