Document the flat wiki namespace and add a dead-link check (ref #816)

contributor_to_wiki_guidelines.md

@@ -1,3 +1,22 @@
 # contributor to wiki guidelines 
 
 ![](https://images.squarespace-cdn.com/content/v1/518f5d62e4b075248d6a3f90/1440708225336-222IWR08EAYDB702LXWE/ke17ZwdGBToddI8pDm48kIXUpvhBrBZPKoLIxiWlZx97gQa3H78H3Y0txjaiv_0fDoOvxcdMmMKkDsyUqMSsMWxHk725yiiHCCLfrh8O1z5QPOohDIaIeljMHgDF5CVlOqpeNLcJ80NK65_fV7S1UW6r-5KZab7F0Ak3dvMWCwb56ceReDoNB9OE6BF3ct6_tN_KfIuurh-w-x7bjNuMJA/image-asset.jpeg?format=1500w)
+
+## GitHub wikis are a flat namespace
+
+Every `.md` file resolves to a URL by filename alone, no matter which
+directory it lives in in this git repo. The `data_files/` directory some
+pages sit in is cosmetic -- `data_files/elements_XML.md` and a hypothetical
+top-level `elements_XML.md` would collide at the same URL. Don't try to
+nest pages by directory; keep filenames unique across the whole repo instead.
+
+## Before adding a link
+
+Run this from the repo root to make sure every `](page_name)` link you added
+actually points at a page that exists:
+
+```sh
+ls *.md data_files/*.md | xargs -n1 basename | sed 's/\.md$//' | sort -u > /tmp/pages.txt
+grep -ohE '\]\(([A-Za-z_][A-Za-z0-9_]*)\)' *.md data_files/*.md | tr -d '()]' | sort -u > /tmp/links.txt
+comm -23 /tmp/links.txt /tmp/pages.txt   # anything printed here is a dead link
+```