Move a playground link to CSS section

[jgpacker]

FlexBox is a CSS feature.
This move is consistent with other files.

What does this PR do?

Improve Repo

[eshellman]

I agree with moving the link. linter wants x to come after u. The broken link is not yours- fixing it is optional.

[borgified]

hi, sorry for jumping in, just wanted to check with @eshellman to see how the new check is looking. is it serving its purpose or is it confusing folks?

[jgpacker]

@borgified from my perspective:

• It can be confusing to have checks from Travis CI and Github Actions at the same time, because each check has logs in a different place (by the way, thanks for the comment @eshellman ).
• As far as I understood, check_urls checks URLs only from files changed in the pull request. I think that's neat! I recommend making this clearer, by changing it's name from check_urls to something like Check URLs from changed files.

[eshellman]

Thank you @jgpacker, both for your PR and your perspective on check_urls.

@borgified I think that running check_urls is really useful, but returning a "fail" is not helpful. added urls get checked for content anyway. can the results of check_urls could be recorded in a separate file somehow?