Add blog post: Defining rule applicability using platform expressions

[jan-cerny]

This PR adds a very short blog post about CPE AL platforms.

[ggbecker]

This is a great blog post. Thank you. I have just minor suggestions otherwise it's good to go.

[jan-cerny]

@ggbecker Thanks for the suggestions!

[ggbecker]

@jan-cerny if you agree with my small fixes I think we can publish this one and announce on every channel we have available that makes sense to announce.

[jan-cerny]

@ggbecker we can't publish it because it's blocked

[ggbecker]

@ggbecker we can't publish it because it's blocked

Aha, I see in the description of the PR now. Thanks. We can wait then.

[jan-cerny]

@ggbecker This PR has been unblocked.

[ggbecker]

@jan-cerny can you please change the date in the _posts/2023-03-08-platforms.md to a recent date so we can publish with today's date?

[jan-cerny]

I have renamed the post to have the current date.

[ggbecker]

I have fixed the CI in another PR, this should be good to go.

[ggbecker]

I have announced on our mailing lists and gitter.