Removes Python 3.0, Notebook 5 mentions from contributor docs (#15710)

* Update contributing.rst

Removes a couple of references to obsolete, end-of-life software

* Update contributing.rst

Fixes intra-page link

---------

Co-authored-by: Michał Krassowski <5832902+krassowski@users.noreply.github.com>

docs/source/developer/contributing.rst

@@ -436,15 +436,10 @@ Notes:
    called something else (such as "python3") then parts of the build
    will fail. You may wish to build in a conda environment, or make an
    alias.
--  Some of the packages used in the development environment require
-   Python 3.0 or higher. If you encounter an ImportError during the
-   installation, make sure Python 3.0+ is installed. Also, try using the
-   Python 3.0+ version of ``pip`` or ``pip3 install -e .`` command to
-   install JupyterLab from the forked repository.
 -  If you see an error that says ``Call to 'pkg-config pixman-1 --libs'
    returned exit status 127 while in binding.gyp`` while running the
    ``pip install`` command above, you may be missing packages required
-   by ``canvas``. Please see `Installing Node.js and jlpm section`_
+   by ``canvas``. Please see the `Installing Node.js and jlpm section`_
    of this guide for instructions on how to install these packages.
 -  The ``jlpm`` command is a JupyterLab-provided, locked version of the
    `yarn <https://classic.yarnpkg.com/en/>`__ package manager. If you have
@@ -477,15 +472,7 @@ Notes:
    However, it takes a bit longer to build the sources, so is used only
    to build for production by default.
 
-If you are using a version of Jupyter Notebook earlier than 5.3, then
-you must also run the following command to enable the JupyterLab server
-extension:
-
-.. code:: bash
-
-   jupyter serverextension enable --py --sys-prefix jupyterlab
-
-For installation instructions to write documentation, please see
+For installation instructions for contributors who want to write documentation, please see
 `Writing Documentation <#writing-documentation>`__
 
 Run JupyterLab