Readme & doc changes for Stage 3

[justingrant]

Here's a few small readme and docs changes now that we're in Stage 3! I wasn't sure if there's particular wording we should be using for our new status, so I tried to keep it simple.

When saving, Prettier made a few whitespace changes that seemed harmless, so I left them in instead of reverting them.

[codecov]

Codecov Report

Merging #1440 (22e5775) into main (3fe5d06) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1440   +/-   ##
=======================================
  Coverage   95.77%   95.77%           
=======================================
  Files          19       19           
  Lines       11113    11113           
  Branches     1801     1801           
=======================================
  Hits        10644    10644           
  Misses        465      465           
  Partials        4        4           

Flag	Coverage Δ
test262	49.04% <ø> (ø)
tests	91.75% <ø> (ø)

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[ptomato]

I think Kevin's suggestion is good. Otherwise 👍 from me.