Improve contribution guide

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

The Contribution Guide could benefit from an overhaul to:

• be more accessible for new developers by reducing required pre-existing knowledge
• mitigate bug risks by documenting processes for common changes
• facilitate code quality by adding code style guidelines

The contribution guide currently only contains descriptions of the technical set-up required to make changes to Parse Server, described in way that assumes a level of pre-existing knowledge about tools and processes that has shown in some cases to discourage new developers from creating PRs. Parse is not just an entry point for new developers for its use as a backend but also when it comes to its development and making first steps into open-source contribution. A more accessible guide benefits both developers and further development of Parse.

Suggested changes

• Add process: Add new Parse Error -> #7075
• Add process: Add new Parse Server configuration parameter -> #7075
• Add: Code style guide
• Add: PR guide (GitHub guide is too verbose and not suitable as a reference for new developers)
• Reformat document to increase readability
• Rewrite guides into step-by-step instructions to help new developers

Please feel free to grab one or propose others.

Start by locating and reading the current Contribution Guide in the Parse Server repository, then review the suggested changes in this issue, including the code style guide, PR guide, readability improvements, and step-by-step instructions. Confirm which unchecked area is still needed with maintainers; done means the selected guidance is documented clearly for new contributors and reflects the existing contribution processes.