Improve testing/demos guide and add benchmarks

[TheJJ]

We have existing documentation for creating tests and demos in doc/code/testing.md.

• [ ] Simplify and improve the guide
• [ ] Add a how-to for benchmarks

[castilma]

doc/code/testing.md points in line 38 to python3 -m openage.testing --help. when I try that. I get /usr/bin/python3: No module named openage.testing.__main__; 'openage.testing' is a package and cannot be directly executed

[TheJJ]

Then testing.md is wrong, as it should be python3 -m openage testing --help without the dot, as the central entry point is the root openage/__main__.py which then delegates the sub-argparser to the openage.testing module.