doc: use readthedocs files structure

Make structure similar to readthedocs tutorial [1].

1. https://docs.readthedocs.io/en/stable/tutorial/

Part of #238

Author: DifferentialOrange

Makefile

@@ -1,4 +1,4 @@
-.PHONY: install test
+.PHONY: install test docs
 install:
 	pip install --editable .
 test: install

README.rst

@@ -121,7 +121,7 @@ To build documentation, first you must install its build requirements:
 
 .. code-block:: bash
 
-    $ pip install -r requirements-doc.txt
+    $ pip install -r docs/requirements.txt
 
 Then run