Update CONTRIBUTING.md file

[Watiriroyalty]

#1264 I noticed that the contributing.md file mentions that every single directory contains an 'index.md' file which contains the markdown text for that directory. However, upon careful scrutiny of the file structure, I noticed that the directories don't actually contain the index.md file. This is an issue because there is no file to update.

[NickSto]

Hi @Watiriroyalty, thanks for taking a look. Yeah, the CONTRIBUTING.md file basically hasn't been updated since we completely rewrote the site so I'm sure it's wrong more often than it's right.

That said, it still should be mostly true that there's an index.md in almost every directory under the content directory. Some directories might have a main.md instead. Those are Inserts which provide content for a corresponding dynamic page in src/pages. But let me know if you have examples that don't follow that rule.