INDEX function has no reference page — slug collides with the section index

[hhimanshu]

Problem

functions.json holds 516 function entries, but only 515 reference pages exist. The
missing one is INDEX.

scripts/gen-docs.mjs slugifies function names to [a-z0-9]+ with dashes, so INDEX →
index. That collides with the generated section index at
content/docs/reference/functions/index.mdx, which wins — the INDEX function page is never
written.

Impact (verified 2026-08-05)

• https://docs.truecalc.app/reference/functions/index → 404
• content/docs/industries/operations.mdx:126 links to /reference/functions/index — a broken
  internal link on a published page
• INDEX is half of the INDEX/MATCH idiom, and comparisons/vlookup-vs-xlookup-vs-index-match.mdx
  exists — so the gap lands on one of the more prominent lookup topics
• The section index page itself renders "516 functions", which overstates what is actually
  documented

Reproduce

node -e "
const fs=require('fs');
const j=JSON.parse(fs.readFileSync('<core>/functions.json','utf8'));
const files=new Set(fs.readdirSync('content/docs/reference/functions')
  .filter(f=>f.endsWith('.mdx')&&f!=='index.mdx').map(f=>f.replace(/\.mdx$/,'')));
const slug=n=>n.toLowerCase().replace(/[^a-z0-9]+/g,'-');
console.log(j.map(f=>f.name).filter(n=>!files.has(slug(n))));
"
# => [ 'INDEX' ]

curl -s -o /dev/null -w '%{http_code}\n' https://docs.truecalc.app/reference/functions/index
# => 404

Suggested fix

Reserve route-colliding slugs in gen-docs.mjs. index is the only collision today, but the
rule should be general — any function name that slugifies to a reserved route needs a suffix
(e.g. index-function) with the section index linking to it under the plain INDEX label.

Worth adding a generation-time assertion that every functions.json entry produced a page, so a
future collision fails the build instead of silently dropping a function.

Acceptance criteria

• A reference page exists for INDEX and is reachable
• /reference/functions links to it under the label INDEX
• industries/operations.mdx:126 resolves (no 404)
• gen-docs fails the build if any functions.json entry yields no page
• The section index count matches the number of pages actually generated

Context

Found while adding a derive-don't-quote rule for function counts — the workspace had been
repeating a stale "484 functions" figure. Deriving the real number from functions.json is what
exposed the missing page.