Fix #4617: quickstart: PROJECT_DIR argument is required

[tk0miya]

refs #4617

[codecov]

Codecov Report

Merging #4621 into 1.7 will decrease coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              1.7    #4621      +/-   ##
==========================================
- Coverage   81.82%   81.48%   -0.35%     
==========================================
  Files         283      283              
  Lines       37345    37502     +157     
  Branches     5847     5849       +2     
==========================================
  Hits        30557    30557              
- Misses       5479     5515      +36     
- Partials     1309     1430     +121

Impacted Files	Coverage Δ
sphinx/cmd/quickstart.py	68.78% <100%> (-0.2%)	⬇️
sphinx/cmd/build.py	0% <0%> (-42.86%)	⬇️
sphinx/environment/adapters/asset.py	81.81% <0%> (-8.19%)	⬇️
sphinx/search/ro.py	0% <0%> (-6.25%)	⬇️
sphinx/search/tr.py	0% <0%> (-6.25%)	⬇️
sphinx/search/en.py	88.23% <0%> (-5.52%)	⬇️
sphinx/builders/websupport.py	58.33% <0%> (-5.31%)	⬇️
sphinx/builders/dummy.py	84.61% <0%> (-3.39%)	⬇️
sphinx/util/rst.py	84.61% <0%> (-3.39%)	⬇️
sphinx/__init__.py	70% <0%> (-3.34%)	⬇️
... and 133 more

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