Update Python language section + Flask tutorial

[luabud]

For microsoft/vscode-python#20267

This PR:

• Updates Install Python section under the Python language topic (cc @cwebster-99)
• Updates the Jupyter section under the Python language topic (cc @minsa110)
• Updates the Flask tutorial to try to clarify how to set FLASK_APP in PowerShell, giving a lot of folks seem to be having issues ehre Error: Could not locate a Flask application.  #1881)