[DOC] Contributing readme and other contributing updates

[MatthewMiddlehurst]

Puts the contributing docs on the front page since we are getting more new contributors.

Adds a request to not @ developers for reviews immediately.

[aeon-actions-bot]

Thank you for contributing to aeon

I have added the following labels to this PR based on the title: [ $\color{#F3B9F8}{\textsf{documentation}}$ ].

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