[python] Make docs searchable

[ivirshup]

This PR addresses https://github.com/chanzuckerberg/cellxgene-census/issues/620

I've re-enabled Sphinx's built-in client side search by:

• Removing the blank template for the search bar
• Removing the line:

html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.11%. Comparing base (36d98e8) to head (f81ac09). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1166 +/- ## ========================================== - Coverage 91.12% 91.11% -0.01% ========================================== Files 77 77 Lines 5892 5886 -6 ========================================== - Hits 5369 5363 -6 Misses 523 523 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1166/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1166/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `91.11% <ø> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ivirshup]

Yeah, it would be nice if we had preview-able doc builds as part of CI. Also a hosted development version of docs.