added pnpm install command to build optimism monorepo

[yasindce1998]

Added pnpm install command for building the optimism repo

Description
When I was trying to build optimism monorepo using pnpm build command right after this command make op-node op-batcher op-proposer . First it was confusing after then I found out I have to use pnpm install command first after then we have to use pnpm build command.
Ref: https://stack.optimism.io/docs/build/getting-started/#build-the-optimism-monorepo

Tests

Since it's documentation changes test cases are not needed.

Additional context

Adding pnpm install command before pnpm build would make installation easier otherwise it'll throw NX not found error also people confuse with yarn/npm install. So this would help beginners to understand these commands properly.

[netlify]

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