Fix relative link to Index Pages in Writing Your Docs help page
Fix relative link to Index Pages in Writing Your Docs help page

The link links to `index_pages` with an underscore but the page as currently rendered has that link as `index-pages` with a hyphen.
waylan/mkdocs,mkdocs/mkdocs,waylan/mkdocs,mkdocs/mkdocs,waylan/mkdocs,mkdocs/mkdocs