Add examples to the `JsonParser` documentation

[33nya]

Purpose

Closes #1972.

• Add examples to the JsonParser specifications to demonstrate the usage of the three parse functions.

Description

• Add examples to the parseString​(String json), parseReader​(Reader reader), and parseReader​(JsonReader reader) functions in JsonParser class.

Checklist

• [x] 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

[33nya]

@Marcono1234 Thank you for your suggestion! It is very clear and helpful. I adjusted the content of the documentation. Please let me know if there is anything that needs to be improved.

[Marcono1234]

Sorry for causing this stalemate situation. @33nya, the only open thing is really minor https://github.com/google/gson/pull/2647#discussion_r1532884565 about the missing space. Unfortunately GitHub does not let me mark my other review comments as resolved.

@eamonnmcmanus, or do you want to merge the changes anyway regardless of that change? If necessary it can also be done as part of another documentation related pull request.

[eamonnmcmanus]

@eamonnmcmanus, or do you want to merge the changes anyway regardless of that change? If necessary it can also be done as part of another documentation related pull request.

OK, I'll merge as-is, and if you or the original author want you can send a follow-up PR.

[33nya]

@Marcono1234 Sorry for causing any inconvenience. I misunderstood and wasn't sure if I could modify the PR after it had been approved, so I didn't make any changes. If it's convenient, could you assist with the adjustments in your current documentation PR? Otherwise, I will make the changes when I modify another documentation PR. Thank you!

[Marcono1234]

@33nya, no problem. I have included that small change in #2654 now, but not sure if / when that will be merged. If you are planning to make some other (documentation) changes anyway, feel free to include it there as well. If not, then it is also not a problem I think; it is probably not worth it to have a separate pull request just for this small change.

[33nya]

@Marcono1234 Alright, I understand. Thank you for your assistance!