Update docs requirements installing

Removes redundant requirements.txt which was also present and
not pinned in setup.cfg. Also updates the docs to reflect the same.
Fixes some other minor bugs to pass doc build tests.

Signed-off-by: Ayan Sinha Mahapatra <ayansmahapatra@gmail.com>

Author: AyanSinhaMahapatra

docs/requirements.txt

@@ -18,18 +18,20 @@ That will create an ``/scancode-toolkit`` directory in your working directory.
 Now you can install the dependencies in a virtualenv::
 
     cd scancode-toolkit
-    virtualenv -p /usr/bin/python3 docs-venv
-    source docs-venv/bin/activate
+    ./configure --dev
+    source venv/bin/activate
 
-Now, the following prerequisites are installed
+Now, the following prerequisites should be installed
 
 - Sphinx
 - sphinx_rtd_theme (the format theme used by ReadTheDocs)
 - docs8 (style linter)
 
 ::
 
-    pip install Sphinx sphinx_rtd_theme doc8
+    pip install -e .[docs]
+
+These requirements are already present in setup.cfg and this installs them.
 
 Now you can build the HTML documents locally::
 
@@ -102,6 +104,10 @@ So run these scripts at your local system before creating a Pull Request::
     ./scripts/sphinx_build_link_check.sh
     ./scripts/doc8_style_check.sh
 
+If you don't have permission to run the scripts, run::
+
+    chmod u+x ./scripts/doc8_style_check.sh
+
 .. _doc_style_docs8:
 
 Style Checks Using ``Doc8``

docs/source/contribute/contrib_doc.rst

@@ -93,7 +93,7 @@ Before you start using Scancode
 
 #. Run ``scancode -h`` to make sure Scancode was installed properly.
    If this shows any Error, refer the `Common Installation Errors Issue <https://github.com/nexB/scancode-toolkit/issues/1837>`_
-   for common errors. The documentation also has tips on :ref:`ide_config`.
+   for common errors.
 
 .. note::