Update API provider documentation

[emnul]

Updated the provider documentation to close #2222

[imaksp]

Hi, new ones like arbitrum, optimism etc are now added:
https://github.com/ethers-io/ethers.js/blob/master/packages/providers/src.ts/infura-provider.ts

I think its better to use same name as Infura like (optimism-mainnet, polygon-mainnet) OR take url & project id as input, it will remove need of this switch case, so it will support all new additions automatically but it would be Breaking change, but can be included in v6, what do you think @ricmoo ?

& in ethers' docs this Infura's docs link can be added:
https://infura.io/docs/ethereum#section/Choose-a-Network

[ricmoo]

I want to make sure we use the same names for all these networks across all providers, so there may be small differences between a given Backend's name and the name used by ethers. In v6, the Network class helps isolate this a bit more, so hopefully it will be less of an issue then. I'm still waiting on getting some of the networks vetted for the ethers default keys, which will work with the getDefaultProvider, which is why it is more important that each Provider uses the same name.

[ricmoo]

Otherwise, merged though.

Thanks! :)