Unify the file extension for YAML files

For a PR, I first searched for all file extensions used by PCL:

git ls-tree -r HEAD --name-only | sed -e 's/.*\.//' | sed -e 's/.*\///' | sort | uniq -c | sort -rn

After calling this command, you will see that there are files with yaml and yml as extension. In general: Both extensions are correct for YAML, but nevertheless I think we should unify it.

Find all YAML files

git ls-tree -r HEAD --name-only | grep -E '.*\.(yaml|yml)'

Result:

.ci/azure-pipelines/azure-pipelines.yaml
.ci/azure-pipelines/build/macos.yaml
.ci/azure-pipelines/build/ubuntu.yaml
.ci/azure-pipelines/build/ubuntu_indices.yaml
.ci/azure-pipelines/build/windows.yaml
.ci/azure-pipelines/docs-pipeline.yaml
.ci/azure-pipelines/documentation.yaml
.ci/azure-pipelines/env.yml
.ci/azure-pipelines/formatting.yaml
.ci/azure-pipelines/release.yaml
.ci/azure-pipelines/tutorials.yaml
.dev/docker/perception_pcl_ros/kinetic_rosinstall.yaml
.dev/docker/perception_pcl_ros/melodic_rosinstall.yaml
.github/FUNDING.yml
.github/ISSUE_TEMPLATE/config.yml
.github/stale.yml
doc/tutorials/content/sources/ground_based_rgbd_people_detection/data/trainedLinearSVMForPeopleDetectionWithHOG.yaml
people/data/trainedLinearSVMForPeopleDetectionWithHOG.yaml

For example, is there a reason why env.yml has yml as file extension but formatting.yaml has yaml?

As the Github names seems to have fixed extensions I suggest to use always yml for all YAML files.

Start with the listed YAML paths and run the provided git ls-tree commands to verify every .yaml and .yml file. Check references in the affected Azure Pipelines, Docker, GitHub, documentation, and data files before renaming them. Done means the chosen extension is used consistently and the repository references remain valid.