Fix "contribute to docs" nesting

[shainaraskas]

Make https://www.elastic.co/docs/extend/contribute/ a top level section and reference it in navigation.yml to avoid the duplicate "Extend and contribute" heading in the sidebar

[shainaraskas]

nobody owns extend-contribute specifically

cc: @georgewallace @bmorelli25

[alexandra5000]

Hey @shainaraskas - I don't know how to triage it, and it seems you're not sure which team it belongs to either 🙂 I'm leaving it as-is for now

[bmorelli25]

Added a parent issue so it doesn't get lost. Whoever ends up owning this can add it to their team board for tracking at a later date.

[georgewallace]

I've created a PR which I think fixes this issue.