Update developing apps guide for archetype 1.4.0-SNAPSHOT

Update developing apps guide for archetype 1.4.0-SNAPSHOT

- Add a new archetype version for Aluminum (1.4.0-SNAPSHOT)
- Update the 'yang-version' to 1.1 and 'revision' to '2021-03-21'
  in 'hello.yang'
- Update HelloWorld example and provide working instructions based on
  1.4.0-SNAPSHOT: implement 'HelloService' with 'HelloProvide' instead
  of adding a separate class 'HelloWorldImpl'.
- Update API explorer instructions to reflect the changes (OpenAPI v3)
  made in Aluminium
- Change indentation to use 4-space tabs: 3-space indentation for
  the numbered list was the source of incorrect indention for Java code
  with extra space
- Update a few capitalization inconsistencies

Signed-off-by: Sangwook Ha <sangwook.ha@verizon.com>
Change-Id: Ic4aa8d4490b6c8cdc73ca33ae946cf28d983dcec