docs: add adr for mongodb selection

docs/architecture-decisions/adr005-database.md

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+# ADR005: Database Selection - MongoDB
+
+## Status
+
+accepted
+
+## Context
+
+We are in the process of selecting a suitable database for the project. In
+general, we need for a database that allows for fast development and changes,
+integrates well with the NestJS framework and is cost-efficient without a vendor
+lock-in (possibility to self-host).
+
+## Decision
+
+We have chosen MongoDB as primary database.
+
+**Fast Development and Changes:** MongoDB's schemaless nature allows for rapid
+development and changes without the need to define a fixed schema upfront. This
+flexibility enables us to iterate quickly and adapt to changing requirements.
+
+**Azure CosmosDB with MongoDB Adapter**: The availability of Azure CosmosDB with
+the MongoDB adapter provides a convenient way to use cloud resources with the
+option to easily opt out if needed. This aligns with the goal of leveraging the
+available Azure credits and taking advantage of cloud-based features such as
+managed backups.
+
+**NestJS Integration**: MongoDB has first-tier support in the NestJS framework,
+which means it is well-integrated and supported out of the box. This simplifies
+the development process and reduces the effort required for integration.
+
+**Widely Spread Database**: MongoDB is a popular NoSQL database with a large
+community, which makes it easy to find resources and contribute to the project.
+
+## Consequences
+
+Next to positive factors already pointed out above, it might be complex to model
+our data in a way that we have the complete safety of a Relational Database.
+Even though MongoDB offers ACID transactions, this is not the "NoSQL way" of
+managing related data. The possible consequence is, that we have to often
+migrate our database, since it takes time to get into the mental model of NoSQL.
+Furthermore, the danger of Azure CosmosDB not providing the latest features of
+MongoDB is also given. The danger of having non-compliant queries is given, but
+rare since Azure CosmosDB widely supports most of the MongoDB specifications.