Add "How to start contributing" page

[Kobzol]

We have discussed this at the All Hands and on Zulip.

This PR adds a new page to Forge that should serve as a landing page for people that would like to start contributing to Rust. I took some content from the previous (very barebones) "Help Wanted" section in the Forge homepage, and from the "What should I work on?" section from rdg, combined them together, and extended them.

After/if this is merged, I'll go edit rdg to remove things that are not strictly related to the compiler from the "What should I work on?" page, and keep only compiler stuff there, as the general documentation should be in the Forge.

r? @davidtwco

CC @jieyouxu

Rendered

[BoxyUwU]

👍 from me about having this in forge instead of the dev guide

[jieyouxu]

Thanks, I think this is a very helpful "entry point" to help discover the contribution guides of not just the compiler / std, but also the toolchain. One tiny nit, but also looks good to me otherwise.