Reactivate devdocs

I just found the devdocs by searching the regular docs (for some reason they are compiled and indexed by the search, even though they do not appear in the menu) and I must say that this is a great piece of documentation that I would've loved to see earlier.

As someone currently developing a Julia package into a C library, several questions that I had (and more) have been answered by reading through it, especially part 1 and part 2.

Thus, IMHO it's really a shame that the docs are not readily findable, since they are not only useful for PackageCompiler.jl devs but also for users such as us who aim to try to understand how compiled libraries work internally and what to expect from them (and what not).

I've seen the discussion in #541 about not wanting to confuse users, but from my perspective, the loss of documentation outweighs the potential gains from hiding it. Thus, would it be possible to reactivate them again?

I'd be happy to create a PR doing so, and to add further safeguards against accidental use. For example, we could only show a single page "Internal" in the nav menu, explain there that this section is only for developers of PackageCompiler.jl, and that these contents should not be used to compile sysimages or libraries directly. Then, we could link to devdocs from this page but keep them hidden from the nav menu by using Documenter.hide.