how-to-start-contributing.md: link directly to std-dev-guide site

[DanielEScherzer]

Instead of to the repo for the guide definition

Rendered

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Fallback group: @Mark-Simulacrum, internal-sites
• @Mark-Simulacrum, internal-sites expanded to Mark-Simulacrum, Urgau, ehuss, jieyouxu
• Random selection from Mark-Simulacrum, Urgau, ehuss, jieyouxu

[apiraino]

thanks