Add an example for CentOS Stream X distro adjustment

[ljavorsk]

There is no mention of CentOS Stream X distro adjustments in the documentation which is now more popular, as RHEL early development is done in those distributions.
I've added a small example, feel free to expand it however you need.

[lukaszachy]

@psss Hm, shouldn't we have similar section in tmt instead? This feels like 'how to write tests' which makes more sense in tmt part.

Either way thank you for the PR @ljavorsk, just need to figure out where to put it.

[ljavorsk]

I've noticed there are already some examples in this doc, that's why I chose it.

[psss]

@ljavorsk, thanks for adding this! @lukaszachy is right that the context adjust example fits a bit more to the tmt documentation. There are two places where it would make sense to mention centos-stream-x values:

• https://tmt.readthedocs.io/en/stable/spec/core.html#adjust
• https://tmt.readthedocs.io/en/stable/spec/context.html#dimension

Thinking about it a bit more, what about rather updating both fmf and tmt documentation to use centos-stream values in the examples instead of centos-x as these are (or soon will be) obsoleted?