docs: clarify PR template instructions

[jimCresswell]

Running "pnpm lint" on a documentation change won't work unless "pnpm build" has previously been run, e.g. on a fresh checkout of a fork after clicking the published docs "edit this on GitHub" button.

Documentation / Examples

• [x] Make sure the linting passes by running pnpm lint
• [x] The "examples guidelines" are followed from our contributing doc