Move the Localnet docker section into separate chapter

Similarly to how we moved the network settings to a separate chapter and how we revamped the Band protocol (#907), I suggest we devote a new "Localnet Development" (dapp/tools/localnet.md) chapter which includes the current content of https://docs.oasis.io/dapp/sapphire/guide#running-a-private-oasis-network-locally

TODO:

[x] mention it could also be used for running Emerald (and replace https://docs.oasis.io/dapp/emerald/writing-dapps-on-emerald#running-a-private-oasis-network-locally with a link to the new chapter)
[x] mention it in the ROFL chapter: https://docs.oasis.io/rofl/app#testing-it-on-sapphire-localnet
[x] replace any links throughout the docs pointing to Sapphire Guide subsection or any links pointing directly to https://github.com/oasisprotocol/oasis-web3-gateway/pkgs/container/sapphire-localnet or https://github.com/oasisprotocol/oasis-web3-gateway/tree/main/docker with the new https://docs.oasis.io/dapp/tools/localnet
[x] mention that minimum requirements for running the Sapphire Localnet image (at least 16GB of memory)

oasisprotocol / docs