Documentation integration (#4653)

* Add documentation under docs/ directory

* Add CI workflow for mkdocs configuration checking

* Add documentation issue template

* update pip-tools?

* Update .gitignore files

* Fix .gitignore rules

* Improve release notes page

* remove references to old repo

readthedocs.yml

@@ -0,0 +1,9 @@
+version: 2
+
+mkdocs:
+  configuration: docs/mkdocs.yml
+
+python:
+  version: 3.7
+  install:
+    - requirements: docs/requirements.txt