[Document-332][docs] Add flinksql Development Guide

[Optimize-330][docs]  Optimize official documents 

[Fix-328][docs]Modify the Chinese and English navigation bar and sidebar

[Document-326][doc] Modify introduction and concept and feature

[Document-324][doc]update registerCenter relevant

[Feature-309][alert] Add Email Alert Type

[Fix-310][gateway]Configure hadoop_conf for Flink

[fix-239][core] Use "==" to judge whether the values of referenced variables are equal. Replace with the equals function.

[fix-239][core] Use "==" to judge whether the values of referenced variables are equal. Replace with the equals function.

[fix-239][core] Use "==" to judge whether the values of referenced variables are equal. Replace with the equals function.

[Feature-312][alert] Add FeiShu Alert Type

[Feature-287][*] Release v0.6.1

modify document

[Fix-294] [admin] Fix to flink conf json bug

[Fix-220][docs] Fix to auto.sh

[Fix-299] [web] Fix to DingTalk form does not display correctly