add Matplotlib to General Purpose packages

[vidurmithal]

Added Matplotlib to the list of General Purpose packages.

Project name: Matplotlib

Project website/repository: https://matplotlib.org/

License: BSD

Checklist for new projects:

• [ x] The project is open-source and accessible.
• [ x] Searched the existing entries to make sure this is not a duplicate.
• [ x] Contains only a single addition (make separate PRs if adding more than one).
• [ x] Added the link to the bottom of the relevant category.
• [ x] Add icon from the media/icon/ folder if applicable (like ![Python](media/icon/python.png) ).
• [ x] Created a new category only if necessary.
• [ x] Checked spelling and grammar.
• [ x] Removed trailing whitespace and periods.
• [ x] Confirm the dash – is not a minus -.
• [ x] Used title-casing (AP style) for project name.