Middleware + docs update

[pacrob]

What was wrong?

Middleware and related docs need cleanup and organization

How can it be fixed?

Noted so far:

Clean up middleware

• Remove middlewares marked Delete in web3/manager.py -> RequestManager -> default_middlewares
• Delete those middlewares from web3/middleware too
• Create tests for both regular and async default middlewares somewhere in tests/core/manager that verify defaults haven't changed
• Add a comment, something like "If you're changing the default middlewares, remember to update tests and docs"

Clean up middleware docs
in docs/middlwares.rst:

• List default middlewares for sync and async providers towards the top
• List all middleware descriptions together, with the Configuring Middleware above or below them
• Any deleted middlewares' docs can be removed.
• HTTPRequestRetry is in the docs but nowhere in the code? Can be removed?

Improve middleware docs

• Document Validation Middleware, any others that are in web3/middleware but not in docs
• Code examples with async middlewares

[reedsa]

HTTPRequestRetry is in reference to the middleware defined here:

web3.py/web3/middleware/exception_retry_request.py

Line 121 in eabd485

def http_retry_request_middleware(