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

Diff for: Makefile

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

Diff for: README.rst

@@ -114,7 +114,7 @@ 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
 
 To build with valid version, `tarantool` package must be installed before
 documentation build.

Diff for: requirements-doc.txt renamed to docs/requirements.txt

@@ -17,7 +17,7 @@
 # sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath('.')), "src"))
 
 # Read package version without importing it
-for line in open(os.path.join(os.path.dirname(os.path.abspath('.')), "tarantool", "__init__.py")):
+for line in open(os.path.join(os.path.dirname(os.path.dirname(os.path.abspath('.'))), "tarantool", "__init__.py")):
     if line.startswith("__version__"):
         exec(line)
         break