[2.0] Finalize docs

[markerikson]

Figure out any docs changes that might be needed and update them:

RTK

• Remove deprecations
• Document new features
• Set up error codes page for RTK
• Document selectSlice
• Look for uses of @reduxjs/toolkit/dist in the docs
• Update TS usage to mention getState() as RootState plus an explicit return type
• Mention isAction in the migration guide
• Update repo README
• Land the Next.js usage docs
• Reselect ParametricSelector removal (and weakMapMemoize as default?)
• Rename migration page and update release note links in existing RCs for completeness)
• Make sure middleware comes before enhancers so we can infer types for getDefaultEnhancers correctly

Core

• Update repo README
• Land the Next.js usage
• Add error codes page to sidebar for core
• Core docs TS changes?
• Copy migration guide
• document isAction and isPlainObject
• Be sure to mention createStore deprecation in the core API ref

Release Notes

• Redux core
  • createStore deprecation
• RTK
  • createStore deprecation
  • Reselect weakMapMemoize, lruMemoize, and dev checks
• Reselect
• React-Redux

[markerikson]

Should add the copy-paste of the extracted errors page, per #3374

[EskiMojo14]

new selectSlice method of slices will need documenting: #3838

[markerikson]

Check uses of augmentation like "@reduxjs/toolkit/dist/query/whatever", per the RTKQ enhancement pages.

[markerikson]

Need to update the Redux templates, too.

[markerikson]

Not 2.0 related, but we should add something to "RTK TS Usage" saying you need to explicitly cast getState() as RootState and have an explicit return type to avoid circular type inference

[EskiMojo14]

as per #3895 we should probably mention isAction in the migration guide

[markerikson]

Land the Next usage docs PR here and in the core site

[markerikson]

Update all repo READMEs with whatever the latest template and descriptions are

[EskiMojo14]

since the migration guide mentions the stability dev check, it should mention the identity function check too