Add secure glossary entry for package type/manager

Why is this change being made?

This MR adds a new Package Managers and Package Types section to the Secure and Protect Glossary of Terms .

The reason for this addition is because we don't currently have a definition for Package Type, and references to it have popped up in recent discussions which have left me confused, for example:

• discussion about what a package type is
• discussion about switching to package types

Screenshots

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