Update CONTRIBUTING.md

Fixes #77215

As mentioned in #77215, the current CONTRIBUTING.md links to the rustc-dev-guide.
Even though the rustc-dev-guide has lots of useful information for contributors,
one is already confused by reading the first line of the current CONTRIBUTING.md.

> To get started, read the [Getting Started] guide in the [rustc-dev-guide].

This line tells the contributor to go and read the rustc-dev-guide. What is
the rustc-dev-guide? What does rustc even mean? These are some of the
questions that went into my head when reading this line as a first time
contributor. By explaining what the rustc-dev-guide is and some platforms
to get help, a new contributor understands what the first step is and the process
is much clearer. The `About the [rustc-dev-guide]` section explains what
the rustc-dev-guide is, what rustc is, and the purpose out of reading the
guide. The `Getting help` section points the user to some places where
they can get help, find a mentor, and introduce themsevles.