doc: generate llms.txt #5789

Job	Run time
Build slim tarball	5m 4s
aarch64-linux: with shared libraries	19m 6s
x86_64-darwin: with shared libraries	42m 51s
aarch64-darwin: with shared libraries	23m 16s
x86_64-linux: with shared libraries	20m 36s
	1h 50m 53s

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