Fix ordering&duplication in contributing docs

[cspotcode]

On the website, "How to Contribute", "Contributing to Arcade", and "How to Submit Changes" all duplicate information and are in the wrong order.

"How to Contribute" is the broadest overview, encompassing non-code ways to contribute. Yet it is nested under "Contributing to Arcade" which is almost solely focused on code changes.

"How to Submit changes" duplicates "Contributing to Arcade" in describing pull requests.

[pushfoo]

"How to Submit changes" duplicates "Contributing to Arcade" in describing pull requests.

This should likely be turned into a link for one of them. For other cases where the content overlap is short, would you be opposed to using includes?

[einarf]

Likely also related to this : #1939

[pushfoo]

This looks completed on development.

[Cleptomania]

Yeah this was definitely handled by the consolidation I did in #1939