doc: Make it easier to find the std docs from the index

[brson]

This is usually the link I want when I come to this page.

r? @steveklabnik

[petrochenkov]

@steveklabnik
Can it be completely reordered? It's on of those slightly inconvenient things I never felt important enough to discuss, but here's an occasion.

My preferred order:
// The first items repeat the items on www.rust-lang.org (Book, Reference, API docs)

1. Getting Started (Book)
2. Specification (Reference)
3. The Standard Library (API docs)
   // Tools
4. Tools (tools are as important as the language and the library, they even have their own team!)
   // Help & Questions
5. Community & Getting Help
6. FAQs
   // Everything else
7. The Error Index
8. Community Translations

[steveklabnik]

@bors: r+ rollup

[bors]

📌 Commit 5ff8e03 has been approved by steveklabnik

[steveklabnik]

@petrochenkov I have no strong feelings about the ordering, that sounds fine to me. Feel free to submit a PR.

[bors]

⌛ Testing commit 5ff8e03 with merge 216f664...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt