Structure contributing guide to support more details

[maxrjones]

Description of proposed changes

There are several open issues that will require adding sections to the contributing guide (e.g., #364, #3313, #3314, #5724). This PR migrates the majority of the contributing guide to the devdocs section of the ReST documentation, keeping only a simple CONTRIBUTING.md in the base of the repository with links to the various sections (following GenericMappingTools/pygmt#1205), which will make the contributing guide easier to navigate as the content expands.

Fixes #

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[PaulWessel]

Looks good!