CI: add release branch testing and documentation workflows #6

Job	Run time
Test jetls executable	6m 15s
Test with `JETLS_DEBUG_LOWERING = true`	9m 48s
Test with `JULIA_NUM_THREADS = 4,2`	9m 2s
Julia 1 - macos-latest - x64	12m 0s
Julia 1 - ubuntu-latest - x64	9m 24s
Test with `JETLS_DEV_MODE = true`	9m 35s
Julia nightly - ubuntu-latest - x64	3m 54s
Julia 1.12 - ubuntu-latest - x64	9m 14s
	1h 9m 12s

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