Docs: improve contributor guide to explain how to work with tickets

[alamb]

Which issue does this PR close?

Part of #7013

Rationale for this change

Recently the question of who is / could be working on what ticket came up in discussions. For example"

• Implement the POSITION function #8969 (comment) from @Lordworms about what tickets were available
• Where @Tangruilin and @simicd ended up working on the same ticket: [task #8213]Port tests in select.rs to sqllogictest #8967

I think it would be good to clarify the contributing guide in this case

What changes are included in this PR?

1. Update the contributing guide to explain the existing decorum: work on what you want, but it is polite to announce on tickets and file new tickets for new behavior

Are these changes tested?

Docs only

Are there any user-facing changes?

[simicd]

Thanks for expanding the contributor guide @alamb, very clear!

[jonahgao]

Suggested change

[good-first-issue](https://github.com/apache/arrow-datafusion/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22)

Moved.

[comphead]

Thanks @alamb would be nice to have a link to contribution.md from the top level page in

https://github.com/apache/arrow-datafusion#contributing-to-datafusion section

[alamb]

Thanks @alamb would be nice to have a link to contribution.md from the top level page in

https://github.com/apache/arrow-datafusion#contributing-to-datafusion section

Excellent call -- done in 94bbca7

[comphead]

nice thanks @alamb