Extend JsonElement documentation

[Marcono1234]

Purpose

Extends the JsonElement documentation, describing various ways to convert from / to JsonElement

Relates to #1972

Description

Extends the JsonElement documentation to describe how to convert JsonElement from / to JSON, but also how to use JsonElement as JSON data and deserializing from / serializing to it.

Maybe the documentation provides too much information now though, potentially confusing users?

A fix for #591 has been implemented in the separate pull request #2659.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

This looks good to me. I think it's valuable, because some aspects of JsonElement usage are not apparent to newcomers.