Standardize and check file names

[marybelzer]

We should standardize on not naming files with underscores in the handbook and while we are at it not allow spaces.
For SEO purposes we need to use hyphens. I.e some-page-name.md rather than some_page_name.md
We need some sort of linting check and we need to go update existing pages.

Handbook content best practices update to reflect this in this PR