Fix sphinx layout to make the search engine working for latest sphinx versions

[sciunto]

Description

closes #4647

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[sciunto]

It's searching forever, same behavior on artifact and locally...

[sciunto]

@scikit-image/core I got some help from sphinx community and the search engine is working again (tested locally). Please check and review.

Example: https://1647-2014929-gh.circle-artifacts.com/0/doc/build/html/search.html?q=canny&check_keywords=yes&area=default

[grlee77]

Thanks for fixing this @sciunto! It is working on the artifact for me as well

[alexdesiqueira]

Thank you @sciunto !