CI: add release branch testing and documentation workflows #5

Job	Run time
Test with `JETLS_DEV_MODE = true`	1m 54s
Test with `JULIA_NUM_THREADS = 4,2`	1m 57s
Julia 1 - macos-latest - x64	1m 40s
Test jetls executable	1m 50s
Julia 1.12 - ubuntu-latest - x64	1m 53s
Julia 1 - ubuntu-latest - x64	1m 53s
Test with `JETLS_DEBUG_LOWERING = true`	1m 53s
Julia nightly - ubuntu-latest - x64	1m 55s
	14m 55s

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