Draft: Rewrite (parts of) CONTRIBUTING.md

What does this MR do?

Splits the Sharkey CONTRIBUTING.md into two files, CONTRIBUTING.sharkey.md (containing Sharkey-specific contribution guidelines) and CONTRIBUTING.md (now simply a copy of upstream with a line at the top redirecting to the .sharkey version). Merge request and issue report templates are also changed to link to the .sharkey version.

CONTRIBUTING.sharkey.md currently contains most of the original content, with some minor changes (e.g. replacing "PR" with "MR"), and a rewrite of the Development section to (hopefully) be clearer.

Things to discuss before marking as ready:

• Should CONTRIBUTING.sharkey.md be stripped down (or rewritten from the ground up) to contain only Sharkey-specific content, or should it continue to be a modified version of upstream?
• Should CONTRIBUTING.sharkey.md be further divided into other files? Currently, it acts as a monolithic document for both guidelines ("your MR should be formatted in a certain way") and development information ("here is how you can develop Sharkey"). Given that the merge request template expects you to have read and agreed to the guidelines, it may be worthwhile separating these topics into two files, allowing readers to clearly understand what the guidelines are, without it being mixed with setup instructions and such.
• Is the "Development" section coherent? My experience on the topic is limited, so there may be some incorrect or incoherent information.
• Are there other sections that warrant rewriting?
• Are there other minor changes that should be made to the wording of the document as a whole?

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