doc/dev describe tool for detecting unused code (#759)

This resolves #759 (closed).

hugo/content/dev/dev/manual_checks.md

++++
+title = "Manual checks"
+weight = 20
++++
+
+## Manual checks
+
+Tests that cannot be automatized should occasionally be run manually.
+
+#### Unused code
+
+Unused code, in particular unused functions, can be detected with the tool
+[xunused](https://github.com/mgehre/xunused).
+
+```bash
+cd build
+cmake .. -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
+vi compile_commands.json # rm source files in 3rdparty and Fit/3rdparty
+xunused compile_commands.json 2>&1 >/dev/null | tee /io/unused.txt
+```
+
+Use judgement: not all currently unused functions must be removed.
+
+#### Static code analysis with clang-tidy
+
+...
+
+#### Coverage
+
+...