learn: change "grow with Rust" to "documentation"

[jsha]

Rationale:

• "grow with Rust" requires some idiomatic, non-literal interpretation, while "documentation" is plainer English.
• When people are looking for documentation, they are usually skimming the site for the literal word "documentation".
• Anecdotally, whenever I type "doc.rust-lang.org" in my URL bar and got redirected to https://www.rust-lang.org/learn, I thought I had made a horrible mistake, because the page I wound up on seemed to have nothing to do with the documentation I was expecting from doc.rust-lang.org. (see also Don't redirect doc.rust-lang.org to www.rust-lang.org/learn rust#77537).