docs(examples): remove redundant examples

[zorzysty]

Removing:

• "custom hooks", because "basic" example uses custom hooks already
• "window focus refetching", because this feature is already turned on by default
• "optimistic updates" because "optimistic updates typescript" already covers this functionality

[codesandbox-ci[bot]]

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Latest deployment of this branch, based on commit 17cc843ae38a8859a9a3f7ba07d3c0cb3664120e:

Sandbox	Source
@tanstack/query-example-react-basic	Configuration
@tanstack/query-example-react-basic-typescript	Configuration

[codecov-commenter]

Codecov Report

Merging #3951 (17cc843) into main (eab6e2c) will increase coverage by 0.45%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3951      +/-   ##
==========================================
+ Coverage   96.36%   96.81%   +0.45%     
==========================================
  Files          45       57      +12     
  Lines        2281     2671     +390     
  Branches      640      784     +144     
==========================================
+ Hits         2198     2586     +388     
- Misses         80       83       +3     
+ Partials        3        2       -1     

Impacted Files	Coverage Δ
src/devtools/devtools.tsx
src/core/queryClient.ts
src/react/useBaseQuery.ts
src/core/onlineManager.ts
src/core/focusManager.ts
src/core/mutationCache.ts
src/devtools/useLocalStorage.ts
src/react/logger.native.ts
src/devtools/utils.ts
src/react/QueryErrorResetBoundary.tsx
... and 92 more

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[TkDodo]

I think we also need to update the docs that the links in the sidebar are not created ?