Add dedicate guide for composite primary keys [ci skip]

[adrianna-chang-shopify]

Motivation / Background

Adds a dedicated guide for composite primary key usage. Most of this content was compiled from other guides / documentation for composite primary keys, including:

• Add composite primary key examples to fixture docs #49156
• Rails Guides: describe composite primary keys (CPK) for migrations and querying #49150
• Add composite primary key examples to Rails guides #49114
• Add additional docs to Query Interface guide about composite primary key models [ci skip] #49188
• [ci skip] Add a section to the Active Record Migration Guide on creating tables with composite primary keys #49186
• Document associations between models with composite primary keys in guides [ci skip] #49182
• Active Record querying guide: add a composite primary key example for find #49153

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]

[nvasilevski]

👍

[nvasilevski]

We seem to be following an existing pattern here but I wonder if Rails should reconsider having such strong message. It's good to encourage reading guides where we think the guides are best presented but also I see no reason to strongly discourage reading them on github if someone finds github more accessible for whatever reason.

Sorry for an off-topic!

[eileencodes]

I think it's because otherwise we get PRs to "fix" the guides on github when they are already correct/fine on the guides website.