Test examples & configuration from documentation

[bhirsz]

We have several code examples in our documentation - either in rst files or doc strings. It's usually command line options or configuration file content.

Unfortunately we had several episodes in the past where we had invalid configuration in our documentation, especially when migration/deprecating something. We need solution to gather such examples and test them.

We will need special markers for it, so the parser will understand if given code example is configuration to be tested or something else which can be ignored. Or at least recognize every line in code blocks that starts with robocop or [tool.robocop.