CI: add release branch testing and documentation workflows #7

Job	Run time
Test with `JETLS_DEV_MODE = true`	3m 27s
Julia 1 - ubuntu-latest - x64	3m 27s
Test with `JULIA_NUM_THREADS = 4,2`	3m 29s
Julia nightly - ubuntu-latest - x64	3m 32s
Test jetls executable	3m 24s
Julia 1.12 - ubuntu-latest - x64	3m 27s
Julia 1 - macos-latest - x64	3m 37s
Test with `JETLS_DEBUG_LOWERING = true`	3m 27s
	27m 50s

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