[stable-2.18] support pip-compile check mode and remove pip-compile workflows from stable branch

[gotmax23]

Support a custom --check flag to fail if pip-compile made any changes so we can check that that lockfiles are in sync with the input (.in) files on all pushes and PRs.

Then, we can also remove the standalone pip-compile workflows from the stable branches as discussed in https://github.com/ansible/ansible-documentation/pull/1920.