Update docs for Check/DeliverTx middlewares

[amaury1093]

Summary

The Check/DeliverTx middleware refactor changes a bunch of things in baseapp, and docs should reflect that.

Proposal

Update docs with the following items. Inspiration can come from https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-045-check-delivertx-middlewares.md

• Go through exising docs, remove mentions of the following outdated items in https://docs.cosmos.network/v0.44/core/baseapp.html

  • baseapp runTx and runMsg (replace with 2.)
  • msgServiceRouter and router in baseapp (replace with 2.)

• Add a new page in Core Concepts called "CheckTx and DeliverTx middlewares" docs: added middleware documentation (#9954) #11445

  • Put context, interfaces from https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-045-check-delivertx-middlewares.md
  • actual logic moved into middlewares
  • each middleware does one thing, and they are easily composable
  • create a tutorial to show how to create a middleware which wraps a function (refactor!: BaseApp {Check,Deliver}Tx with middleware design #9920 (comment)) (like a paragraph or a subsection)
  • Explain the most important middlewares:
    • runMsg txHandler (to replace the paragraph in baseapp)
    • Gas
    • Event
    • recovery (so that we can remove https://docs.cosmos.network/v0.44/core/runtx_middleware.html)
    • store branching (so that we can remove the paragraph in https://docs.cosmos.network/v0.44/core/context.html)

• Add new page in the Migrations subsection for migration docs Antehandlers -> Middlewares

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[amaury1093]

Re-opening, there are still 2 docs pieces