☂️ Update ./contributing/ resources

With the addition of the `/contributing` directory, we have a space to add even more documents teaching and empowering others to write code in Consul. One important thing to note is that contributing/development docs are distinct from user facing web docs.

Related Issues:

- [x] [Identify places to improve on docs INTERNALS.md](https://github.com/hashicorp/consul/issues/7039)
- [ ] [Enhance contributing/INTERNAL.md RPC documentation](https://github.com/hashicorp/consul/issues/7038)
- [ ] [Write an ACL internals document](https://github.com/hashicorp/consul/issues/4837)
- [ ] [Improving filtering API docs](https://github.com/hashicorp/consul/issues/6049)

_This is an umbrella issue. Umbrellas do not have their own implementation, but rather are a way to link together various related issues and their implementations under a common goal. Github automatically cross-references issues and PRs when mentioned, so adding an issue to an umbrella provides an easy way to loosely associate similar but independent tasks. They also provide a location for meta-level discussion of a goal that is larger than a single task. Umbrellas can be kept open for long-running living documentation for ongoing goals, or closed out and held for reference later if the goal has a clear stopping point._

Start by reviewing the ./contributing/ directory and the linked issues, especially INTERNAL.md RPC documentation, the ACL internals document, and filtering API docs. This umbrella issue has no standalone implementation or completion criteria; work is done through one of its related issues.

Written by the indexing model from the issue text.