Standardize links to lesson pages

[kelly-sovacool]

from #166 (comment) by @MrFlick:

It's still not possible for the client to know for sure that https://umcarpentries.org/intro-curriculum-r/05-r-markdown/ and https://umcarpentries.org/intro-curriculum-r/05-r-markdown/index.html are the same resources. It would be nice to standardize all URLs either to use the "index.html" file or to leave it off and end the path with a slash. But this just means that values are repeated for the identical pages with different URLs.

I think my preference is to not include index.html at the end of paths, but it doesn't really matter, as long as we pick one and be consistent about it.