New pipeline

[Spomky]

This PR is a proposal for a new pipeline management.
It is derived from what is done with e.g. web-token or webauthn.

• Github Community Standards complementary files
  • CONTRIBUTING: .github/CONTRIBUTING.md file
  • Pull request template: .github/PULL_REQUEST_TEMPLATE.md
  • New issue templates
• dependabot rules
• stale rules
• Makefile in favor of composer scripts
• Pipeline:
  • .github/workflows/merge-me.yml : auto merge PR from dependabot (depends on CI status)
  • .github/workflows/release-on-milestone-closed.yml : this GH Action is very useful and allows seamless branches and tags mgmt.
    • Branches shall be e.g. 2.0.x, 2.1.x, 3.0.x...
    • Milestones shall have the same version numbering, but x is an actual number (e.g. 2.0.1, 2.0.2, 2.1.0...).
    • When a milestone is closed (e.g. 2.7.0), this action will add a tag 2.7.0 on the branch, create minor and minor branches if deemed necessary (2.8.x and 3.0.x) and create a PR for merging 2.7.0 to 2.8.x.
    • All PR and issues should point to the correct milestone. This will help creating nice release notes such as this one: https://github.com/web-auth/webauthn-framework/releases/tag/4.2.0

Please note that some files require changes to be in line with this project.
In addition, the GH Actions require some project or organization secrets:

• "GITHUB_TOKEN": ${{ secrets.GITHUB_TOKEN }}
• "SIGNING_SECRET_KEY": ${{ secrets.SIGNING_SECRET_KEY }}
• "GIT_AUTHOR_NAME": ${{ secrets.GIT_AUTHOR_NAME }}
• "GIT_AUTHOR_EMAIL": ${{ secrets.GIT_AUTHOR_EMAIL }}

To be discussed.