docs: add architectural decision record (ADR) template

Author: ValuedMammal

docs/adr/template.md

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+# [short title of solved problem and solution]
+
+* Status: [proposed | rejected | accepted | deprecated | … | superseded by ADR-1234]
+* Authors: [list everyone who authored the decision]
+* Date: [YYYY-MM-DD when the decision was last updated]
+* Targeted modules: [which crate or module does this change target]
+* Associated tickets/PRs: [PR/issue links]
+
+## Context and Problem Statement
+
+[Describe the context and problem statement, e.g., in free form using two to three sentences. You may want to articulate the problem in form of a question.]
+
+## Decision Drivers <!-- optional -->
+
+* [driver 1, e.g., a force, facing concern, …]
+* [driver 2, e.g., a force, facing concern, …]
+* … <!-- numbers of drivers can vary -->
+
+## Considered Options <!-- numbers of options can vary -->
+
+#### [Option 1]
+
+[example | description | pointer to more information | …]
+
+**Pros:**
+
+* Good, because [argument …]
+
+**Cons:**
+
+* Bad, because [argument …]
+
+#### [Option 2]
+...
+
+#### [Option 3]
+...
+
+## Decision Outcome
+
+Chosen option: "[option 1]", because [justification. e.g., only option, which meets k.o. criterion decision driver | which resolves force force | … | comes out best (see below)].
+
+### Positive Consequences <!-- optional -->
+
+* [e.g., improvement of quality attribute satisfaction, follow-up decisions required, …]
+* …
+
+### Negative Consequences <!-- optional -->
+
+* [e.g., compromising quality attribute, follow-up decisions required, …]
+* …
+
+## Links <!-- optional -->
+
+* [Link type] [Link to ADR] <!-- example: Refined by [ADR-0005](0005-example.md) -->
+* … <!-- numbers of links can vary -->