Generate documentation as part of the build process

[serge-sans-paille]

There's a doc directory with these two files docgen.zig langref.html.in but no rule in CMakeLists.txt to use them to generate a proper documentation.
Any hint on how to do so, something I could use to submit a patch would be great!

Thanks

[mikdusan]

from linux ci script:

zig/ci/azure/linux_script

Lines 76 to 79 in 236db62

	release/bin/zig test ../lib/std/std.zig \
	--override-lib-dir ../lib \
	-femit-docs=release/docs/std \
	-fno-emit-bin

[g-w1]

Also, run

zig build-exe docgen.zig
./docgen `which zig` langref.html.in langref.html

to generate the docs.

[mikdusan]

ah yes, and I think this also works to gen langref.html

zig build docs