[Doc][Relay] Add VM doc (#3188)

* [Doc][Relay] Add VM doc

* Add Apache header

* Apply suggestions from code review

Co-Authored-By: Steven S. Lyubomirsky <slyubomirsky@gmail.com>
Co-Authored-By: 雾雨魔理沙 <lolisa@marisa.moe>
Co-Authored-By: Logan Weber <36520469+weberlo@users.noreply.github.com>
Co-Authored-By: Zhi <5145158+zhiics@users.noreply.github.com>

* Junru's comment

* More fix

* More fix

* More fix

* last fix

* Apply suggestions from code review

Co-Authored-By: 雾雨魔理沙 <lolisa@marisa.moe>

* Apply suggestions from code review

Co-Authored-By: Logan Weber <36520469+weberlo@users.noreply.github.com>

* Add code links

* Remove unused bp

* Update docs/dev/virtual_machine.rst

Co-Authored-By: Logan Weber <36520469+weberlo@users.noreply.github.com>

* Explain TODO

* Yong's comment

Co-Authored-By: Yong Wu <55wuyong@163.com>

* Comment