feat: fee grant module doc

[Bidon15]

### Tasks
- [x] add and test celestia-app portion of guide
- [ ] add and test celestia-node portion of guide

[jcstein]

The feegrant module is a module of the Cosmos-SDK. It can be thought of similarly to how on Ethereum, users can interact with a contract that has a “paymaster” who will pay for the transactions or mints for other users.

[rootulp]

This gist may help. It was not obvious to me that if you want a fee granter to pay fees for a grantee, then the commands the grantee invokes (via CLI) should not specify a --fees CLI flag.

[jcstein]

process in docs could go:

1. set up a new light node with no funds
2. set up fee grant in celestia-app
3. send pfb from the light node celestia-node