[docs] add conventions page to the developer docs #5465

Job	Run time
Julia 1 - windows-latest - x64 - pull_request	21m 40s
Julia nightly - ubuntu-latest - x64 - pull_request	21m 9s
Julia 1 - ubuntu-latest - x64 - pull_request	19m 26s
Julia 1.6 - ubuntu-latest - x86 - pull_request	20m 47s
Julia 1.6 - ubuntu-latest - x64 - pull_request	16m 34s
	1h 39m 36s

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