sql tuning: add explain walkthrough (#4685)
sql tuning: add explain walkthrough (#4685)

* sql tuning: add explain walkthough

* Update explain-walkthrough.md

* Apply suggestions from code review

* Update explain-walkthrough.md

* Apply suggestions from code review

Co-authored-by: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>
Co-authored-by: Zhang Jian <7819760bb8f9dc30316d73d6b3588211c475d795@gmail.com>

* Apply suggestions from code review

Co-authored-by: Zhang Jian <7819760bb8f9dc30316d73d6b3588211c475d795@gmail.com>

* Update explain-walkthrough.md

Co-authored-by: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>
Co-authored-by: Zhang Jian <7819760bb8f9dc30316d73d6b3588211c475d795@gmail.com>
pingcap/docs-cn,pingcap/docs-cn