Add CI tests with docutils 0.15.2 and 0.16
As docutils has two recent releases 0.15.2 and 0.16 after 0.14 https://pypi.org/project/docutils/#history, which currently we are using on CI tests.
Minor fixes
- rst web site URL updated to https://docutils.sourceforge.io/rst.html
Blocked by !19 (merged)
Edited by Takuya Noguchi