Add 'Issues assignment' section to the Contributing Guidelines

[julien-deramond]

Description

This PR suggests adding a new section to our Contributing Guidelines named 'Issues assignment' that explains that we don't assign issues to contributors and why.

Motivation & Context

As maintainers, we often get asked to assign issues to contributors. We don't do that for several reasons.
IMO it's important to explain why we don't do that in our Contributing Guidelines to avoid frustration and confusion for contributors.
On our side, it will allow us having a link to point to when we get asked to assign issues.

On my side, if it is merged, I'll update my GitHub Saved reply to point to this section that I'll be able to share here as a comment right after if you're interested.
Having the same kind of answers would bring consistency and coherence to our answers.

Improvement 1

I wanted to add to this PR a message at the end of all newly created issues (via the issues template) to point to this section but it's not possible to do it. Some sources that confirm it:

• https://github.com/orgs/community/discussions/5631
• https://github.com/orgs/community/discussions/32948
• https://github.com/orgs/community/discussions/5310
• https://github.com/orgs/community/discussions/48397

So I'll try to provide a GitHub Action later, as a second step, to add this message to all newly created issues.
Edit: done via #38622

Improvement 2

Still IMHO, at some point, it would be nice to have a kind of "pr welcome" label to add to issues when they are ready to develop for maintainers or contributors, and/or a kind of "core team only" if this can't be treated outside because of the complexity or any other reasons. It would be a good way to help contributors to find issues to work on and would bring some clarity on who can do what to help us.

Type of changes

• Enhancement (non-breaking change)

Checklist

• I have read the contributing guidelines
• 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
• (N/A) I have added tests to cover my changes
• (N/A) All new and existing tests passed

Live previews

• https://github.com/twbs/bootstrap/blob/e8bed76c5c45917898ef6a936027a38bcca263a9/.github/CONTRIBUTING.md#issues-assignment

[mdo]

Thanks for this!