Add minimal testing devdocs #2176

Job	Run time
NodeJS for prerender	1m 4s
Julia 1.6 - ubuntu-latest - x64	4m 18s
Documentation: html	1m 7s
Julia 1.6 - macos-latest - x64	6m 42s
CSS for HTML themes	47s
PDF/LaTeX backend	3m 22s
Documentation: pdf	2m 49s
Julia 1.6 - windows-latest - x64	13m 42s
Julia 1 - ubuntu-latest - x64	4m 1s
Linkcheck: online tests	2m 3s
Julia 1 - macos-latest - x64	5m 33s
Linkcheck: Documenter manual	4m 53s
Julia 1 - windows-latest - x64	4m 39s
Prettier (JS code formatting)	11s
Julia nightly - ubuntu-latest - x64	4m 28s
Julia nightly - macos-latest - x64	8m 8s
Julia nightly - windows-latest - x64	5m 48s
Julia 1 - ubuntu-latest - x86	4m 34s
	1h 18m 9s

Provide feedback