Improve quickstart: gradient section

[mariaschuld]

Moves the "interface" section to the top (since this is used by beginners, while the current "gradient" section is more advanced).

Adapts the language to move from "interface" to "automatic differentiation framework".

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #2564 (3f36a96) into master (8c7a589) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2564   +/-   ##
=======================================
  Coverage   99.57%   99.57%           
=======================================
  Files         243      243           
  Lines       19471    19471           
=======================================
  Hits        19388    19388           
  Misses         83       83           

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[josh146]

💯