Draft: Resolve "Add an introduction to nmaas Guides"

Closes #75 (closed)

docs/guides/introduction.md

+# nmaas Guides
+
+The Guides section is dedicated to hosting step-by-step instructions on how to perform common tasks from the nmaas web interface (the nmaas Portal). The following guides are currently available:
+
+- [Application Manager Guide](application-manager-guide.md) - targeted at users with the application manager role. It outlines the required steps to add a new version of an existing application in the nmaas catalog.
+- [Domain Admin Guide](domain-admin-guide.md) - targeted at users with the `Domain Admin` role. It describes the steps to add or remove users to/from an existing domain.
+- [User Guide](user-guide.md) - targeted at regular users. It provides a comprehensive overview on how nmaas can be used, starting from requesting a new domain and registering a user account, to deploying application instances, accessing them, reconfiguring them, and finally deprovisioning application instances which are no longer needed.
+
+Please note that these guides are aimed at nmaas users interacting with the platform through its web interface. Administrators who would like to deploy a new nmaas instance should have a look at the [nmaas Installation Guide](../self-hosted-nmaas/introduction.md).
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