Improve available CONTRIBUTING information and guidelines

[jrfnl]

Description

Recreation of upstream PR squizlabs/PHP_CodeSniffer#3830 with some additional tweaks:

CONTRIBUTING: move file to .github directory

... which is nowadays the more common location for this type of file.

Includes updating the link in the README file.

Ref: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

Composer: add scripts for contributors

These scripts should make contributing to PHPCS more straight-forward as they document common CI checks and allow for running these locally.

CONTRIBUTING: rewrite the contributing guide

This is a rewrite of the CONTRIBUTING.md file to document expectations and opportunities for contributors.

It also answers some common questions previously received from contributors.

Suggested changelog entry

N/A

Related issues/external references

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement