Update getting started and reference docs

[choldgraf]

This is a quick update on some documentation, with these main changes:

• A bit more docs around the fancy new "merge output streams" feature, so folks are more likely to find and use it :-)
• Moves some stuff in start/ into a dedicated configuration reference so that it's easier to find
• Updates the start/ guide with a few more steps people can take
• Makes a few top-level section titles "imperative" so that we cut back on wordiness (and partially inspired by https://diataxis.fr/how-to-guides/)

This is a follow up to #364

[codecov]

Codecov Report

Merging #365 (a2694cc) into master (47b33d5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #365   +/-   ##
=======================================
  Coverage   87.34%   87.34%           
=======================================
  Files          12       12           
  Lines        1367     1367           
=======================================
  Hits         1194     1194           
  Misses        173      173           

Flag	Coverage Δ
pytests	87.34% <ø> (ø)

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[choldgraf]

thanks for the review @mmcky - I think this one is good to go unless there are objections - if I got the description wrong I am happy to iterate in subsequent PRs

(note that the codecov tests seems to be wonky because of a missing credential or something, don't think it's related to this PR)

[mmcky]

thanks @choldgraf

[mmcky]

@choldgraf just updated from the master branch -- all looks good. I think this is good to 🚢

[mmcky]

@chrisjsewell @choldgraf I will merge this later this afternoon so we can merge: jupyter-book/jupyter-book#1448