CI (Buildkite): Update all rootfs images to version v3.1, add some documentation, and reorganize/rename/move a bunch of configuration files

[DilumAluthge]

No description provided.

[codecov]

Codecov Report

Merging #41698 (75c2771) into master (073af4a) will decrease coverage by 0.92%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #41698      +/-   ##
==========================================
- Coverage   81.27%   80.34%   -0.93%     
==========================================
  Files         350      350              
  Lines       74074    74137      +63     
==========================================
- Hits        60203    59565     -638     
- Misses      13871    14572     +701     

Impacted Files	Coverage Δ
base/intfuncs.jl	92.70% <100.00%> (+0.05%)	⬆️
base/compiler/validation.jl	5.50% <0.00%> (-83.49%)	⬇️
base/compiler/ssair/verify.jl	1.48% <0.00%> (-59.26%)	⬇️
base/compiler/stmtinfo.jl	58.33% <0.00%> (-33.34%)	⬇️
base/compiler/parsing.jl	42.85% <0.00%> (-28.58%)	⬇️
stdlib/Test/src/Test.jl	63.77% <0.00%> (-24.51%)	⬇️
base/promotion.jl	47.23% <0.00%> (-23.62%)	⬇️
base/pkgid.jl	46.15% <0.00%> (-23.08%)	⬇️
base/compiler/typelimits.jl	61.06% <0.00%> (-20.80%)	⬇️
base/expr.jl	78.60% <0.00%> (-17.04%)	⬇️
... and 105 more

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