Rework the pull request template to improve documentation and avoid confusion

[b3n]

Description

The existing PULL_REQUEST_TEMPLATE.md proposed in the this repository does not seem to be used.

Looking at the recently closed PRs, either the check-boxes for the reviewer(s) were simply ignored, either the whole template was entirely skipped.
In both cases, there is barely any references to an existing issue!

I propose to rework the existing pull request template in order to:

1. insist on the need for an issue to be referenced;
2. optionally give a better explanation about why and how to fill the form.

Motivation

In short:

• better document why we are changing things;
• reduce the confusion generated by the current template;

Acceptance criteria

Two options. Replace the existing template by:

1. a minimal version inspired by the one;
2. a longer version inspired by this general one.

Additional information

See LeastAuthority/it-ops/issues/145 for the context on this matter.

[b3n]

Mentioning @ewanas here since he's the author of the original commit (6e239b4).

[b3n]

I'm in favor of option 2, so I'm opening #243 for review...
Let's fall back on #242 if we feel this is a better path for this project.