Add check and docs for TEMPLATES APP_DIRS=False.

[tim-schilling]

The toolbar requires at least one DjangoTemplates TEMPLATES
configuration needs to have APP_DIRS set to True.

Closes #1491

[codecov]

Codecov Report

Merging #1498 (be21a99) into main (6cc6265) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1498      +/-   ##
==========================================
+ Coverage   85.72%   85.84%   +0.12%     
==========================================
  Files          35       35              
  Lines        1891     1893       +2     
  Branches      272      273       +1     
==========================================
+ Hits         1621     1625       +4     
+ Misses        190      189       -1     
+ Partials       80       79       -1     

Impacted Files	Coverage Δ
debug_toolbar/apps.py	92.72% <100.00%> (+0.27%)	⬆️
debug_toolbar/panels/profiling.py	91.07% <0.00%> (+1.78%)	⬆️

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