ISSUE-3603 Some API doc updates for ErrorPolicy, FetchPolicy, and ObservableQuery

[MikaelCarpenter]

References

• Issue API doc issues #3603
• zen-observable
• ApolloClient docs
• ReactApollo docs

What

• Ensure the current links for ErrorPolicy and FetchPolicy work
  • currently the ApolloClient docs try to link to these policies via {@link FetchPolicy} in watchQueryOptions.ts
  • there is currently no place under the ApolloClient docs that describe these policies. Instead, they're described in the ReactApollo docs
  • my initial solution is to add a redirect in docs/_config.yml that will take you from the ApolloClient docs to ReactApollo docs.
• Add a mention of zen-observable to the ObservableQuery docs
  • I'm not sure how much info should be added here. So all I really do is add a link to the repo directly enderneath the ObservableQuery header.

Why

There were some comments in Slack that pointed out an issue with the links and the fact that knowing about extending zen-observable could be helpful.

Fixes #3603.

[MikaelCarpenter]

It looks like there was a timeout during the install-npm step in circleci. If anyone has any info on why that might be, let me know!

[hwillson]

Thanks very much @MikaelCarpenter - LGTM!