Update plug-in naming convention in documentation

[Stef3st]

In issue #1429 the term extensions was renamed to plug-ins to avoid confusion. The term usage still needs to be applied to the documentation.

Acceptance criteria:

• [ ] Rename Extensions.md to Plug-ins.md and update the references to this file
• [ ] Rename the term extensions to the above mentioned file to plug-ins
• [ ] Use the term plug-ins in the Home.md file and update the reference.
• [ ] Update Glossary.md to use the term plug-ins instead of extensions
• [ ] Update _Sidebar.md to use the term plug-ins instead of extensions