Migrate to pnpm v10 as package manager

[fflaten]

Replaces yarn v1 (classic) with pnpm v10 as new package manager. pnpm is a popular and modern package manager:

• It's really fast - and will soon become even faster with v12, a v11-compatible rewrite in Rust
• Uses a global store that reuses packages between projects, saving disk-space
• Great security features for less chain-supply risk etc. Even though it's not really important for this static site.

Pinning pnpm v10 for now because Dependabot-support for v11 is still in beta. Might go straight to v12 when ready.

One of the security features is blocking postinstall-scripts in packages. Currently configured:

• @swc/core: Allowed. Used by docusaurus. Downloads a OS-dependent native binary.
• core-js: Denied. Just a funding notice.

Fix #275

[fflaten]

@nohwnd I just noticed Netlify has been using npm run build and not yarn build until now (and pnpm build here). If this is approved, do you want to update in UI or should I commit a netlify.toml in the repo to configure it?