Limit CI/CD push trigger to dev branch only.

[ghost]

What does this PR address?

• Reduces duplicated CI/CD checks.
• Prevents pipelines from running on arbitrary branches.

Closes #1290.

[github-actions]

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[bradjohnl]

@andrzej-casper While it's good to remove duplicated CI runs, running the CI on every branch had the advantage of giving constant feedback to developers and contributors.

If @adam and @ipopescu are fine with having feedback on builds only on PRs and after a successful merge, then I am fine with this change.

If you'd still like to have the continuous feedback on every push to every branch, then we have to find some other way to prevent duplicate builds I guess

[ipopescu]

@andrzej-casper @bradjohnl @ACStoneCL, I think having the CI/CD checks for open PRs, and after a PR merge to the dev branch, is sufficient. Also, it is good to run the docs locally when we're creating documentation. I forget the rationale behind "continuous feedback on every push to every branch". Perhaps it was nice to have during the docs restructuring, when we had 10+ branches in progress at one time, but if it is causing side effects now, we can reduce the checks.

[ghost]

@bradjohnl, please note that for any changes requiring CI/CD, contributors can use PR drafts. I just did that with #1305 to run preview deployment for some stale branch.