Document zig std in langref.html

[ExeVirus]

I'm very new to zig, and I just got my bearings on where to get most information and where to reach out in the community. Most things were obvious except for one:

zig std is not mentioned really anywhere; instead we point users to the online documentation at master:

zig/doc/langref.html.in

Line 356 in ec60156

The <a href="https://ziglang.org/documentation/master/std/">Zig Standard Library</a> has its own documentation.

I propose that this section of langref.html be updated to inform users that they can access their specific version of the zig std documentation locally via zig std.