Add template engine ADR

docs/decisions/0039-use-apache-velocity-as-template-engine.md

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+---
+nav_order: 39
+parent: Decision Records
+---
+
+# Use Apache Velocity as template engine
+
+## Context and Problem Statement
+
+We need to choose a template engine for [custom export filters](https://docs.jabref.org/collaborative-work/export/customexports) and [AI features](https://github.com/JabRef/jabref/pull/11884).
+
+A discussion of template engines was also in [one of the JabRef repos](https://github.com/koppor/jabref/issues/392).
+
+A discussion was raised on StackOverflow ["Velocity vs. FreeMarker vs. Thymeleaf"](https://stackoverflow.com/q/1459426/10037342).
+
+## Decision Drivers
+
+* It should be fast.
+* It should be possible to provide templates out of `String`s (required by the AI feature).
+* It should have short and understandable syntax. Especially, it should work well with unset fields and empty `Optional`s.
+
+## Considered Options
+
+* Apache Velocity
+* Apache FreeMarker
+* Thymeleaf
+
+## Decision Outcome
+
+Chosen option: "Apache Velocity", because "Velocity's goal is to keep templates as simple as possible" ([source](https://stackoverflow.com/a/1984458/873282)). It is sufficient for our use case.
+Furthermore, Apache Velocity is lightweight, and it allows to generate text output. This is a good fit for the AI feature.
+
+## Pros and Cons of the Options
+
+### Apache Velocity
+
+- Main page: <https://velocity.apache.org/>.
+- User guide: <https://velocity.apache.org/engine/devel/user-guide.html>.
+- Developer guide: <https://velocity.apache.org/engine/devel/developer-guide.html>.
+
+Example:
+
+```text
+You are an AI assistant that analyses research papers. You answer questions about papers.
+
+Here are the papers you are analyzing:
+#foreach( $entry in $entries )
+${CanonicalBibEntry.getCanonicalRepresentation($entry)}
+#end
+```
+
+* Good, because supports plain text templating.
+* Good, because it is possible to use `String` as a template.
+* Good, because it has simple syntax, and it is designed for simple template workflows.
+* Good, because it has a stable syntax ([source](https://stackoverflow.com/a/1984458/10037342)).
+* Bad, because it is in maintenance mode.
+
+### Apache FreeMarker
+
+- Main page: <https://freemarker.apache.org/index.html>.
+- User guide: <https://freemarker.apache.org/docs/dgui.html>.
+- Developer guide: <https://freemarker.apache.org/docs/pgui_quickstart.html>.
+
+Example:
+
+```text
+You are an AI assistant that analyzes research papers. You answer questions about papers.
+
+Here are the papers you are analyzing:
+<#list entries as entry>
+${CanonicalBibEntry.getCanonicalRepresentation(entry)}
+</#list>
+```
+
+* Good, because supports plain text templating.
+* Good, because it is possible to use `String` as a template.
+* Good, because in active development.
+* Good, because it is powerful and flexible.
+* Good, because it has extensive documentation ([source](https://stackoverflow.com/a/1984458/10037342)).
+* Neutral, because it has received some API and syntax changes recently ([source](https://stackoverflow.com/a/1984458/10037342)).
+* Neutral, because FreeMarker is used for complex template workflow, which we do not need in JabRef.
+
+### Thymeleaf
+
+- Main page: <https://www.thymeleaf.org/>.
+- Documentation: <https://www.thymeleaf.org/doc/tutorials/3.1/usingthymeleaf.html>.
+
+Example:
+
+```text
+You are an AI assistant that analyzes research papers. You answer questions about papers.
+
+Here are the papers you are analyzing:
+[# th:each="entry : ${entries}"]
+[(${CanonicalBibEntry.getCanonicalRepresentation(entry)})]
+[/]
+```
+
+* Good, because supports plain text templating.
+* Good, because it is possible to use `String` as a template.
+* Good, because it has [several template modes](https://www.thymeleaf.org/doc/tutorials/3.1/usingthymeleaf.html#what-kind-of-templates-can-thymeleaf-process), that helps to make HTML, XML, and other templates.
+* Good, because it is powerful and flexible.
+* Neutral, because the API is a bit more complex than the other options.
+* Bad, because the syntax is more complex than the other options. Especially for text output.
+
+## More Information
+
+As stated in [the template discussion issue](https://github.com/koppor/jabref/issues/392), we should choose a template engine, and then slowly migrate previous code and templates to the chosen engine.
+
+<!-- markdownlint-disable-file MD004 -->