🔧 Add sphinx-immaterial to doc theme builds

[codecov-commenter]

Codecov Report

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

Project coverage is 89.13%. Comparing base (9412892) to head (cb2d960).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #190 +/- ## ======================================= Coverage 89.13% 89.13% ======================================= Files 11 11 Lines 957 957 ======================================= Hits 853 853 Misses 104 104 ``` | [Flag](https://app.codecov.io/gh/executablebooks/sphinx-design/pull/190/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/sphinx-design/pull/190/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `89.13% <ø> (ø)` | | 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=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.