Update documentation for external authentication SSO Backend to get a proper install

[syyna]

Environment

• Python version: 3.8.5
• Nautobot version: 1.02b

Steps to Reproduce

1. Follow steps to install SAML SSO backend in https://nautobot.readthedocs.io/en/latest/configuration/authentication/sso/
2. Run into several issues due to incomplete documentation of dependencies to install

Expected Behavior

Following the doc should get me to the intended next step

Observed Behavior

Following the doc led me to an error

Fix

These are the proper steps for SAML dependencies for external authentication:
Using sudo outside the venv

1. sudo apt install -y libxml2-dev libxmlsec1-dev libxmlsec1-openssl pkg-config

Switching to user nautobot and installing within the venv.
2. sudo -iu nautobot
3. source bin/activate
4. pip install testresources
5. pip install --upgrade pip setuptools wheel
6. pip install python3-saml

This is tested on ubuntu 20.04 with the base nautobot install using the nautobot install docs.

[jathanism]

Thanks @syyna for the report! 👀

[dgjustice]

This is related to issue #149