Add "Documentation coverage" GSoC project idea

gsoc2025/index.md

@@ -303,6 +303,33 @@ Enhance Swift Testing's reporting of test results to the console/terminal. Consi
 
 - [Stuart Montgomery](https://github.com/stmontgomery)
 
+### Documentation coverage
+
+**Project size**: 90 hours (small)
+
+**Estimated difficulty**: Intermediate
+
+**Recommended skills**
+
+- Basic proficiency in Swift.
+
+**Description**
+
+Enhance Swift DocC's experimental documentation coverage feature to write coverage metrics in a new extensible format that other tools can read and display. 
+Define a few types of metrics—for example Boolean (has documentation: true/false), Fraction (2/3 parameters are documented), Percentage, etc.—for this format. 
+Explore ideas for what documentation coverage information would be useful to emit. Explore ideas for how another tool could display that coverage information. 
+
+**Expected outcomes/benefits/deliverables**
+
+- Land the documentation coverage output format changes for the experimental feature in DocC.
+- Submit a pitch to the community and the Documentation Workgroup to formally enable the documentation coverage feature in DocC.
+- Summarize your effort with a demo of the new metrics and examples of how another tool could display that information.
+
+**Potential mentors**
+
+- [David Rönnqvist](https://github.com/d-ronnqvist)
+
+
 ### Example project name
 
 **Project size**: N hours