CI: add release branch testing and documentation workflows #4

Job	Run time
Test with `JETLS_DEV_MODE = true`	8m 18s
Julia 1 - ubuntu-latest - x64	8m 14s
Julia nightly - ubuntu-latest - x64	3m 57s
Test with `JETLS_DEBUG_LOWERING = true`	8m 15s
Julia 1.12 - ubuntu-latest - x64	8m 18s
Test with `JULIA_NUM_THREADS = 4,2`	8m 14s
Test jetls executable	6m 36s
Julia 1 - macos-latest - x64	8m 23s
	1h 0m 15s

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