Improve Quickstart: New section on inspecting circuits

[mariaschuld]

Make a new section advertising features that inspect circuits.

The examples are taken mostly from the release notes.

We decided not to add tests, as there is a autotest function https://docs.python.org/3/library/doctest.html.

[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 #2566 (8d73caf) into master (0ac1c69) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2566   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         243      243           
  Lines       19596    19596           
=======================================
  Hits        19515    19515           
  Misses         81       81           

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[josh146]

💯 Happy for this to be merged once @albi3ro's comments are addressed!

[albi3ro]

Great to have this! I think it will definitely help out a lot of users.