[Dev][Developer Tools] Add root AGENTS.md file

### Describe the enhancement requested

Create an `AGENTS.md` file in the project root directory that explains any shared concepts that are not language-specific such as information around issue creation, using `pre-commit` etc. This can contain links to individual `AGENTS.md` files in specific implementations where more focused information lives.

If generating this using AI, one suggestion would be to look at the [new contributors guide](https://arrow.apache.org/docs/developers/guide/index.html) and then tailor any AI-generated output based on balancing conciseness with informativeness.

Another potentially helpful use of AI here could be to look at recent PRs by first-time contributors in the past few months, and assess whether there are any commonalities in typical feedback received as this may provide a clue as to what kind of information would have the most impact on improving AI-generated PR quality.

### Component(s)

Developer Tools