documentation for openage cli

The openage cli grants access to quite some features.

$ openage --help
usage: openage [-h] [--version]
               {main,game,test,convert,convert-file,codegen} ...

free age of empires II engine clone

positional arguments:
  {main,game,test,convert,convert-file,codegen}

optional arguments:
  -h, --help            show this help message and exit
  --version, -V         print version info and exit

Further help can always be viewed by openage $mode --help, but what we need is a guide to which modes are useful for what, in one document:

We should create a documentation page in doc/cli.md that serves as a landing page on how to use the CLI. From there, links to other documents and their sections can be placed, for example:
openage test should link to doc/code/testing.md where the invocations are further explained.

Likewise, such links or explainations should be added for other modes of operation, e.g. the convert and convert-file, and so on.

Start with the CLI modes listed in the issue and review doc/code/testing.md for the existing test-command guidance. Create doc/cli.md as a landing page explaining which modes are useful and linking to the relevant documentation for test, convert, convert-file, and the other modes. Done means the main CLI usage is covered in one navigable document.