Remaining AC3 documentation updates

[hwillson]

This PR houses and tracks the remaining AC3 documentation work that needs to be completed for launch.

TODO:

• Remaining local state updates
  • Handling @client fields with field policies
    • add a mutation example
    • add a merge function example
  • Using reactive variables to track @client state
  • Integrating @client into remote queries
  • Handling async operations with field policies and reactive variables
  • Handling @client fields with the cache
  • Using @client fields as variables
  • Client side schema
  • Refactor content layout
    • Combine updating/querying local state
    • Adjust all subsections to be l2's to show in right hand menu
    • Deprecate in local resolvers title
• Remove broken Meteor docs
• Consider cache policies vs. field policies specific naming
• Remove all Apollo Component docs examples
• cache.modify
  • Be sure to demonstrate both updating and removal
• cache.identify
• Migration guide for moving from local resolvers to cache policies
• @apollo/client/core, @apollo/client/utilities, @apollo/client/testing, @apollo/client/cache entry points
• canRead and readField helpers
• cache.evict(id, fieldName) changes to support specific entity field eviction
• readQuery / writeQuery API updates (e.g. options.id)
• Remove all FragmentMatcher, HeuristicFragmentMatcher, and IntrospectionFragmentMatcher references
• Explain new cache console.log warnings, what they mean, how to resolve them
• How to address dangling references
• Fall back to cache-first after cache-and-network or network-only (8ce2b4f)
• Update all read / merge docs to make sure they reflect the final version of the API
• Replace all mentions of node-fetch with cross-fetch (fdcd519)
• Silencing broadcasting (Allow silencing broadcast for cache update methods. #6288)
• Review migration guide to make sure all important/common path breaking changes are mentioned
• Consider updating the intro path to mutations to replace the new default approach of using cache.modify with a readQuery / writeQuery approach instead (then briefly mention cache.modify as a more advanced alternative)
• Make sure @live support removal is mentioned somewhere
• Update the migration guide
  • Mention local state management changes, including reactive variables
  • Outline how to migrate from local resolvers to cache policies
  • Decide if we want to highlight important breaking changes, or refer to the CHANGELOG (as we're duplicating details)
• Consider adding a InMemoryCache section under API - CORE explaining main API cache functions and options
• Make sure all cacheRedirects mentions/examples have been replaced with type policies
• Refactor all pagination documentation
  • Document new pagination helpers (Deprecate fetchMore updateQuery function, and provide basic field policy helper functions #6464 and Implement a relayStylePagination field policy helper function. #6465)
• Deprecate fetchMode's updateQuery, replacing with field policies (Deprecate fetchMore updateQuery function, and provide basic field policy helper functions #6464)
• Remove broken Performance > Query splitting section
• Decide if we want to write a new Performance > Query splitting section using read and merge functions
• Replace cache.makeVar references to direct makeVar imports (Make makeVar a global function instead of a method of InMemoryCache. #6512)

[hwillson]

@StephenBarlow @benjamn @jcreighton I'll take this out of draft as I think there's starting to be a bit much to review all at once. I'll continue working on the outstanding todo items in separate PR's off of this one. As far as reviewing goes, commit by commit should be a reasonable path.

One quick thing to note @stephen - I don't think there's any value in reviewing the Local resolvers (DEPRECATED) page. I've removed a few sections that are no longer needed, but other than that we're going to be removing local resolvers completely in the next major AC version. Maybe just a quick review to make sure the deprecation warning message is okay?

Oh, and one more thing - Netlify doc previews are currently broken for all AC PR's. I'll follow up with @trevorblades on this.

Thanks all!

[StephenBarlow]

Green to go for tomorrow! I'll try to get some eyes on the preview docs today, and we can of course follow up quickly with any hiccups folks encounter.

[hwillson]

Awesome @StephenBarlow - I'll get this merged first thing in the AM. Thanks very much!

[StephenBarlow]

@hwillson Hold off on merging tomorrow until our esteemed colleague @trevorblades is prepared to push a complementary Netlify change roughly simultaneously. Otherwise, all of the existing 3.0-beta doc URLs out there will be broken for a time.