Allow fetching special files in the documentation

[EWright212]

In #1514, we added a symlink so that the docs would show up on the demo app …but
symlinks aren't supported on Windows and was causing a problem when trying to bundle.

This instead adds a lookup table of files not in docs/ which allows us to render those.

This partially fixes #1693, in allowing Administrate itself to be bundled without a symlink but
other dependencies have the same problem.

[nickcharlton]

Nice! A few comments to make it a little bit more Ruby-like.

I think too we might want to look at trying to test this out in: spec/features/documentation_spec.rb

[pablobm]

I'm trying to test this, but I'm having issues getting things installed on Windows (surprise). I'm trying something new; I'll report back.

[pablobm]

As mentioned, I've been trying to get this working on Windows, but have failed so far :-/ How did you install things on Windows yourself? I have tried this so far:

• RailsInstaller: latest version of Ruby there is 2.3, and Administrate requires at least 2.4.
• WSL+Ubuntu on Windows: symlinks appear to work.
• RubyInstaller: I can't get bundle install to complete, first for lack of symlinks, then due to the gem kgio not being compatible.

So it appears that you installed things in some way that allowed you to run bundle install, but yet failed later due to lack of symlinks. Am I correct?

[nickcharlton]

I think this is good to go now.

To answer @pablobm's question, if we flip the messaging of this PR to be about what it really is, vs. what motivated it is perhaps a better way of putting it. We were trying to fix #1693, where Administrate was using a symlink and breaking things. I don't think we can do much about dependencies (apart from going to their repos and opening PRs to fix them there).

We could perhaps rename this to be: "Allow fetching special files in the documentation", with a body like:

In #1514, we added a symlink so that the docs would show up on the demo app …but
symlinks aren't supported on Windows and was causing a problem when trying to bundle.

This instead adds a lookup table of files not in docs/ which allows us to render those.

This partially fixes #1693, in allowing Administrate itself to be bundled without a symlink but
other dependencies have the same problem.

[pablobm]

Right, I had misunderstood something. I thought this was about running the Administrate dev environment, which would require running bundle install in the folder of the Administrate gem. However, this is about using Administrate in a different project. I can reproduce this now.

OK, let's try this. I can't really test it properly because the problem doesn't appear if I install the gem from the GitHub repo (gem 'administrate', git: '...') as opposed to from the gem proper (just gem 'administrate'). The error does point at that symlink though, so I think it's worth trying this.