Validate HTML generator produces unique files

[erichoracek]

If you have a website with a SectionID case that has the same name as a page markdown file, right now whichever one ends up in the output is nondeterministic. This change adds a validation step to the HTMLGenerator to ensure uniqueness.

[codewithhart]

Interesting find. I just spent way too long figuring out why the top-level *.md files weren't updating. From what I've seen around the web...is the correct behavior to have a SectionID case match the .md file?