[docs]: Flesh out CONTRIBUTING.md

[saehejkang]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fleshed out the CONTRIBUTING.md docs. This is taking reference from the CONTRIBUTING.md file, in the containerization repo, but with modifications.

Added new sections to help first time contributors with tips on how to get started with issues, how to ask for help, and who to ask for help.

Closes #880 as it was preferred to not add a CODEOWNERS file, but instead add notes to where contributors can find a list of maintainers to ping.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[katiewasnothere]

I think the goal for pointing to the contributing guide in containerization was to have a single source of truth for both repos on our policies around contributing.

If we're adding content here, I would prefer we keep the redirect at the top of the file to containerization, but imo a lot of the advice here is also useful in containerization. So maybe we add changes there instead.

[saehejkang]

I was thinking of adding the changes I made in this PR (if approved), to the containerization repo as well. However, it might become difficult to maintain, if we are consistently making changes to either docs (and we expect them to be exactly 1:1). If the guide in the other repo is the "single source of truth", I can make the changes there instead.

[katiewasnothere]

I'm adding a comment in #880 about this, just to keep all of those thoughts in one place.

[saehejkang]

This PR is closed and will be completed in apple/containerization#426