Update documentation

[dai-chen]

Update development guide with our new project structure (new Gradle submodule and ./gradlew command changes) and Git workflow we follow (the new develop branch): https://github.com/opendistro-for-elasticsearch/sql/blob/master/docs/developing.rst

[dai-chen]

JDK version in README is out-of-date. Should be JDK 14+.

[dai-chen]

There was confusion reported in issue #607. So we need to clarify field type is nested: https://github.com/opendistro-for-elasticsearch/sql/blob/master/docs/user/beyond/partiql.rst#example-employees

[Hokwang]

I am waiting this doc updates.

[Hokwang]

@dai-chen any updates?

[dai-chen]

@dai-chen any updates?

Hi @Hokwang , I just created local branch for this. Could you let me know which update are you waiting for?

[Hokwang]

@dai-chen it is #607.

[dai-chen]

@dai-chen it is #607.

I see. Will try to get it updated soon. Thanks!

[Hokwang]

@dai-chen Thanks for your great doc work. but I think you need to update one more thing. https://opendistro.github.io/for-elasticsearch-docs/docs/sql/workbench/
actually I read this page at first time. I think repo is https://github.com/opendistro/for-elasticsearch-docs/.

[dai-chen]

@Hokwang thanks for reminding! yeah, that's our doc repo for all ref manual. we will push all latest doc later.