add/refine some user documentations

[zz-jason]

update documents in:

• https://github.com/pingcap/docs-cn
• https://github.com/pingcap/docs

including but not limited to the following items in the check list:

• refine document about system variables @XuHuaiyu sql: add some new tidb specific system variables docs-cn#852
• add document about the implementation of hash aggregate operator @XuHuaiyu Add document about the implementation of hash aggregate operator #7609
• add document about the physical aggregate operator chosen in planner @winoros
• add document about the physical join operator chosen in planner @winoros
• refine document about the explain statement @zz-jason
• add document about how to investigate slow queries @zz-jason

[morgo]

I've added an example for EXPLAIN in pingcap/docs#599.