link to std-dev-guide from the landing page

[jyn514]

i.e add a new top-level "how do i contribute to std?" section to https://rustc-dev-guide.rust-lang.org/getting-started.html

cc https://discord.com/channels/273534239310479360/592856094527848449/1120138588336689274

[zahash]

Hi, can i do this?

[zahash]

#1783

[zahash]

can this issue be closed now?

[tshepang]

indeed