[Docs]: Update docs to comet 0.38

[tac0turtle]

Summary

comet 0.38 introduced vote extensions and Finalize block, we should update the docs to capture the changes and update the notion of begin/end block in the docs as well

• https://docs.cosmos.network/v0.50/building-modules/beginblock-endblock
• add docs for how modules can use vote extension
• mention changes in logic, beginBlock is not the beginning block any longer but prepareproposal is now
• update https://docs.cosmos.network/main/core/baseapp

[alexanderbez]

@tac0turtle which documentation in particular? We should aim to make issues as detailed as possible.

[tac0turtle]

edited the comment above