Improve branch documentation (also for packages/testing contributions)

Could you please apply your changes on https://github.com/ethereum/execution-specs/tree/forks/amsterdam (the default branch) instead of the `devnets/bal/3`?

We need better docs for branch usage/practices here, sorry about that. The `devnets/bal/3` branch gets created from `eips/amsterdam/*` and will never get merged into `forks/amsterdam`. We'll rebase `devnets/bal/3` on `forks/amsterdam` to get your fixes.

More background: The `eips/amsterdam/*` branches get merged individually into `forks/amsterdam` once they've been SFI'd and we're confident that they'll hit mainnet. The `devnets/*` branches mainly just allow us to build specific interim devnet specs that aren't necessarily the spec that'll hit mainnet.

_Originally posted by @danceratopz in https://github.com/ethereum/execution-specs/issues/2274#issuecomment-3943151276_