Write a guide and redo examples

[jsdw]

• Write a guide which covers getting set up to use Subxt and then each of the major areas
• Revamp all examples; all examples will be included as part of the guide.

We'll aim to cover some of the other doc issues we have open in the guide (I'll close them since we will address them as part of this):

• The ConstantsClient API needs better documentation #811
• Add an example for overriding types in the subxt macro #612 (we generally want to just document the macro usage nicely)
• Add an example of using OfflineClient to create and sign a transaction #611 (this is a good "general" example use case)
• Add an OfflineClient example to the repo #728 (OfflineClient example)
• Create FAQ to address common issues #393 (some sort of FAQ or guide for common issues is probably a good idea!)
• Document requirements for the task executor #367 (a good note to document that Tokio is likely required if jsonrpsee is used)
• How to Create an Extrinsic for any Module （E.g write proof on chain and retrive it back for POE dapp） #258 (We'll cover this; building and submitting extrinsics)