doc: compile tutorial programs

The programs in `doc/examples` are not compiled currently, so let's
compile them if the `docs` and `examples` options don't disallow it.

`tutorial4.c` needs a small modification to avoid `-Wfloat-conversion`.

Additionally, install them if `installed_tests` is not disabled.

doc/examples/meson.build

@@ -0,0 +1,23 @@
+tutorials = [
+  'tutorial1',
+  'tutorial2',
+  'tutorial3',
+  'tutorial4',
+  'tutorial5',
+  'tutorial6',
+  'tutorial7',
+]
+
+if not get_option('examples').allowed()
+  subdir_done()
+endif
+
+foreach c : tutorials
+  executable(
+    c,
+    sources: c + '.c',
+    dependencies: [ pipewire_dep, mathlib ],
+    install: installed_tests_enabled,
+    install_dir: installed_tests_execdir / 'examples',
+  )
+endforeach