Warn if files in Pages of @content, @index do not exist

[goerz]

While writing the test for JuliaDocs/DocumenterCitations.jl#48, I stumbled on some issues with the Pages attribute in @content and @index blocks:

1. There is no warning if a file listed in Pages does not exist. I'd even argue that in strict mode (default), referencing a file that does not exist should throw an exception.
2. Pages in @content and @index blocks does not have normpath applied to it, which is inconsistent with @autodocs, as already noted in About non canonical bibliography inconsistencies (to autodocs) DocumenterCitations.jl#22 (comment). This is especially pernicious in combination with (1), as people are probably somewhat likely to just write unix path separators (/), and then have their documentation build be subtly wrong (but not fail) on Windows.

Arguably, @autodocs should also warn if its Pages contains an entry that doesn't match anything.

Both of the above problems are handled in DocumenterCitation's @bibliography block, leading to the inconsistencies shown in this example:

https://github.com/JuliaDocs/DocumenterCitations.jl/pull/48/files#diff-94253cd814e34a8eed4ea72a3fe7c963d73ea2149f6f080d42a8f933545c8bf1

[mortenpi]

I'd even argue that in strict mode (default), referencing a file that does not exist should throw an exception.

I agree that this would be ideal. I am wondering how we should handle new build-breaking errors now that we have promised not to break things for a while (i.e. whether this would constitute a breaking change or not).

[laborg]

As a user, I would warmly accept this “breaking” change as it would help me avoid providing incorrect documentation (missing pages!).

[goerz]

Personally, I wouldn’t think of this as “breaking”, but I suppose it’s debatable.