Add operators SQL references (#844)

* Add subqueries doc and fix select

* add explain docs

* fix

* add information schema doc

* fix to explain

* Add operators doc

* fix

* fix links

* Remove changes to separate pr

* Remove unexisting reference

* Update website/docs/reference/sql/operators.md

Co-authored-by: peasee <98815791+peasee@users.noreply.github.com>

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Co-authored-by: Luke Kim <80174+lukekim@users.noreply.github.com>
Co-authored-by: peasee <98815791+peasee@users.noreply.github.com>

Author: 3 people