[docs] update GEMINI.md with Nexus test instructions (#12542)

This commit updates GEMINI.md to include instructions for building and
running Nexus tests. Nexus is a test framework that enables faster and
more scalable network simulations within a single process.

The updated documentation provides:
- A brief overview of the Nexus test framework.
- The command to build Nexus tests using 'tests/nexus/build.sh'.
- Commands to run all Nexus tests or a specific test using
  'tests/nexus/run_nexus_tests.sh'.
- A reference to 'tests/nexus/README.md' for further details.

GEMINI.md

@@ -69,6 +69,30 @@ To run the tests after building with the simulation preset:
 ctest --preset simulation
 ```
 
+### Nexus Tests
+
+Nexus is a test framework that allows simulating multiple OpenThread nodes within a single process, enabling faster and more scalable network simulations.
+
+**Build Nexus tests:**
+
+```bash
+top_builddir=nexus_test ./tests/nexus/build.sh
+```
+
+**Run all Nexus tests:**
+
+```bash
+top_builddir=nexus_test ./tests/nexus/run_nexus_tests.sh
+```
+
+**Run a specific Nexus test:**
+
+```bash
+top_builddir=nexus_test ./tests/nexus/run_nexus_tests.sh <test_name>
+```
+
+For more details, see [tests/nexus/README.md](tests/nexus/README.md).
+
 ## Development Conventions
 
 ### Code Style

tests/nexus/README.md

@@ -19,7 +19,6 @@ Nexus is a test framework for OpenThread testing.
 To build Nexus test cases, the `tests/nexus/build.sh` script can be used:
 
 ```bash
-mkdir nexus_test
 top_builddir=nexus_test ./tests/nexus/build.sh
 ```