Source links to standard library functions missing in Julia manual

[mortenpi]

E.g. anything on the LinearAlgebra or Tar pages (and most others). Related to JuliaLang/julia#43307 (although we now just omit the source links, rather than pointing to the wrong place).

At least for some we need to create a way to point to (the right commit) of an external repository (e.g. Tar.jl). I am pretty sure the paths in the docstring metadata point to the Julia repository, as the code gets copied there during build. LinearAlgebra I am less sure about right now, as it still lives in the Julia repo, but clearly something is also going wrong there.

Slightly related to #1808. However, properly fixing this will likely involve some more refactoring of the way we handle these URLs in Documenter, and lead to some behavior changes. So I would like to hold off on 0.28 until we have this.

[mortenpi]

Just wanted to put it here that there is progress on this issue on these branches:

• https://github.com/JuliaDocs/Documenter.jl/tree/mp/julia-stdlibs
• https://github.com/mortenpi/julia/tree/mp/stdlib-docs

I also have a demo build (https://mortenpi.eu/juliadocs-demos/issue-1975-fix), but I need to find the time to do another pass on that branch.