Point within Contributing.md to our docs at docs.pi-hole.net

[yubiuser]

What does this PR aim to accomplish?:

Replace the custom text of Contributing.md with a link to our documentation. The text was a 1:1 copy anyway, I think it's better to have only one source-of-truth in case we want to change some guidelines.

Cave: targeting master on purpose.

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[rdwebdesign]

I like the idea of having just one Contributing.md, but I'm undecided if this should be on the docs or if we should add it to pi-hole/.github/ repository and remove the individual files from all repos.

Reference:
https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/creating-a-default-community-health-file

[yubiuser]

I like the idea of having just one Contributing.md, but I'm undecided if this should be on the docs or if we should add it to pi-hole/.github/ repository and remove the individual files from all repos.

Great idea!