docs/mempool: Document current implementation

[hvanz]

The current documentation of the mempool is quite incomplete.

New content

• [x] High-level architecture
• [x] Interaction and dependencies with other modules/reactors
• [x] Main data structures: clist, cache, ...
• [ ] Messages
• [x] Main processes/functions
• [ ] Concurrency

DoD

• All main components of the current implementation are documented.
• The documentation can be used as a guide by CometBFT developers and users to understand the current implementation.
• Dependencies to other modules are identified.

[hvanz]

This document with a diagram covers:

• High-level architecture
• Interaction and dependencies with other modules/reactors
• Main data structures: clist, cache, ...
• Main processes/functions

It doesn't cover:

• Messages
• Concurrency

[cason]

The messages and channels used by the reactor are documented here: https://github.com/cometbft/cometbft/blob/main/spec/p2p/messages/mempool.md

It is worth double checking and possibly fixing this documentation.