Add CI that checks that pull request readiness checklist is there in the PR.

[varun-jaiswal17]

Closes #291
Closes #292

Adds a CI workflow that validates the Pull Request Readiness Checklist on every PR.

Checks :

• PR description is not empty
• ### Pull Request Readiness Checklist section is present
• All 6 required checklist items from the PR template are present (not deleted/modified)
• All checklist boxes are checked - [x] before merging

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The PR is proposed to the proper branch
• There is a reference to the original bug report and related work
• The feature is well documented and sample code can be built with the project CMake