docs: add .nvmrc and point contributor setup at it

The accessibility check (`make a11y` / `npm run check-a11y`) runs pa11y-ci's
sitemap fetch, which breaks on Node 24.17.0+ (node-fetch "Premature close").
Add .nvmrc pinned to the same known-good 24.16.0 CI uses so local runs match
CI, and replace the vague "Node.js v20+" guidance in README and CONTRIBUTING
with a pointer to it.

Author: LauraLangdon

.nvmrc

@@ -0,0 +1 @@
+24.16.0

CONTRIBUTING.md

@@ -13,7 +13,7 @@ Thank you for investing your time in contributing to our project!
 
 - You can preview your changes to the site either:
   - Locally, as follows:
-    - Install Node.js v20+ and run `npm install`
+    - Install Node.js and run `npm install` (use the version in [`.nvmrc`](.nvmrc); run `nvm use`)
     - Run `make serve` (or `npx myst start`)
   - Via deploy preview (branch PRs only—not available for fork PRs):
     - Open a PR from a branch in this repo
@@ -81,7 +81,7 @@ Here are some resources to help you get started with open source contributions:
 
     - Commit locally as you progress (`git add` and `git commit`)
     - To preview your changes:
-      - Install Node.js v20+ and run `npm install`
+      - Install Node.js and run `npm install` (use the version in [`.nvmrc`](.nvmrc); run `nvm use`)
       - Run `make serve` (or `npx myst start`)
     - To check accessibility: run `npm run build` then `npm run check-a11y`
     - To make sure your code passes linting and formatting checks, you can:

README.md

@@ -4,7 +4,7 @@ The [UC OSPO Network](https://ucospo.net) website, built with [MyST](https://mys
 
 ## Prerequisites
 
-- [Node.js](https://nodejs.org) v20+ and npm
+- [Node.js](https://nodejs.org) and npm. Use the version in [`.nvmrc`](.nvmrc) (run `nvm use`); Node 24.17.0+ currently breaks the accessibility check due to an upstream regression.
 - [Python](https://www.python.org) 3.10+ (optional, for pre-commit hooks)
 
 ## Local development