Improve CONTRIBUTING.md formatting and instruction

[hyperupcall]

This improves the wording and formatting of CONTRIBUTING.md. Also since the file contains some really good instructions for contributing to the repository (and there doesn't seem to be issues anymore), the issues that are many years old can now be closed:

• closes How to submit a schema PR? #363
• closes Add tutorial for adding schema to the repository #71
• closes [Question] How to schema custom package.json configs #811
• closes Reference local schema #2267

Additionally, to reduce noise and to make it easier to peruse existing issues, I would also like to close the following issues related to HTTP(S) since they are now fixed or irrelevant:

• closes Fix https on schemastore.org #1225
• closes when pulling from the schema store receive 403 Web App Unavailable  #1223
• closes Certificate of https://schemastore.org/ invalid AGAIN (recurring issue) #1480

Is that OK? These issues are very old and have been fixed for a while.

[GerryFerdinandus]

Thanks