docs: adopt spec structure in x/payment and x/qgb

[liamsi]

Summary

The readme in https://github.com/celestiaorg/celestia-app/blob/91e35a90c00d49b7fc302f356220d8d830b8fa3b/x/payment/spec/docs.md is a great start. IMO, we should structure this the same way as all sdk modules. This will help auditors and SDK devs understand the modules we are writing.

Also, we must ensure that these docs are and stay up to date (especially as we might add some params like max ODS size etc).

Example: https://github.com/cosmos/cosmos-sdk/tree/e7066c4271ff3d33dc426dc6313c82a1201ae3c6/x/distribution/spec#contents

[rootulp]

It looks like Cosmos SDK modules have migrated away from this spec/* directory structure in favor of a README.md at the root of the module directory (e.g. distribution/README.md). Related cosmos/cosmos-sdk#13758 . To conform with the new style:

x/blob

• move current spec/docs.md to README.md in module root chore: move blob README #1057
• update docs (started in Update docs for payment module #948)

x/qgb

• write new docs in x/qgb/README.md docs: add qgb module readme #1052

[rootulp]

See https://github.com/cosmos/cosmos-sdk/blob/main/docs/spec/SPEC-SPEC.md#common-layout for an updated guide