Add PR template

[mdo]

• Wait for issue template feedback
• Iterate on content here
• Update commit authors to include @GeoSot

[julien-deramond]

There is maybe an unwanted side effect with checkboxes here.
Considering the PR filled this way:

It will be considered as a checklist by GitHub and will be displayed like that in the PRs list:

"7 of 10 tasks" may be troubling or frustrating and can't be fixed by the user since they are not real tasks but choices.

If you find this comment relevant, we could try another approach like that:

<!-- What types of changes does your code introduce? Uncomment the following types that apply. -->

<!-- - Bug fix (non-breaking change which fixes an issue) -->
<!-- - New feature (non-breaking change which adds functionality) -->
<!-- - Refactoring (non-breaking change) -->
<!-- - Breaking change (fix or feature that would change existing functionality) -->

[julien-deramond]

Wait for issue template feedback

Just out of curiosity did you have any feedback of the issue template?

On my side I would love having access to this PR template. Tried it in several PR description and it helps me to fight against laziness and force me to add more details 😄 Really like the "Live preview" part which can help the reviewer to save some time without having to search the modification in the documentation.

[mdo]

I here you on your feedback about the checkboxes and progress... if you have thoughts on how to rewrite this to avoid that, I'm down to review. Otherwise I'm inclined to close this as stale :).

[GeoSot]

Do we have any knowledge, if github is planning to support form templates (yml) on PRs too?

[julien-deramond]

Note: Issue forms are currently in beta for public repositories on GitHub.com only.

I suppose we'll have to wait some time unfortunately.
@mdo Please let this PR open a little, I'll try to propose something.

[julien-deramond]

Here is my proposal:

### Related issues

<!-- Please link any related issues here. -->

### Description

<!-- Describe your changes in detail -->

### Motivation & Context

<!-- Why is this change required? What problem does it solve? -->

### Types of change

<!-- What types of changes does you code introduce? -->
<!-- Please remove the unused items in the list -->

- Bug fix (non-breaking which fixes an issues)
- New feature (non-breaking change which adds functionality)
- Refactoring (non-breaking change)
- Breaking change (fix or feature that would change existing functionality)

### Checklist

<!-- Go over all the following points, and put an `x` in all the boxes that apply. -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're here to help! -->

- [ ] I have read the [contributing guidelines](https://github.com/twbs/bootstrap/blob/main/.github/CONTRIBUTING.md)
- [ ] My code follows the code style of the project _(using `npm run lint`)_
- [ ] My change introduces changes to the documentation
- [ ] I have updated the documentation accordingly
- [ ] I have added tests to cover my changes
- [ ] All new and existing tests passed

### Live previews

<!-- Please add direct links where your modifications can be seen in the documentation -->

* https://deploy-preview-{your pr number}--twbs-bootstrap.netlify.app/

Whatever the versions chosen (this one or the previous one), IMHO we should put one in place and see how we and the users are going to use it. The PRs description are often incomplete or even empty. On top of that we would save some time if everyone took the time to just insert a link to the documentation where the impact could be seen; or the regressions measured.

[GeoSot]

I've updated it, including the 'Previews' section.

As I initially transformed the "Types of changes" section, to comments, I realized that "Checklist" section also may not be checked in total (most of the PRs don't need documentation changes), so we will end up again to the same issue of falsie uncompleted tasks.

So till github announce the support of form templates (yml), let's try it as it is

Feel free to make any changes, but as @julien-deramond wrote, it will be handy to start using it as soon as possible

[mdo]

👍🏻 Let's try it!