New logo, dark mode on landing page

[bkoelman]

Added dark mode to the documentation landing page. The selected theme (light/dark/auto) is stored at the same local storage key as docfx, so they share the setting.

Updated the JsonApiDotNetCore logo, because it became unreadable (black text on black background) in dark mode.

Closes #1291.

QUALITY CHECKLIST

• [x] Changes implemented in code
• [x] Complies with our contributing guidelines
• [ ] N/A: Adapted tests
• [x] Documentation updated

[codecov[bot]]

Codecov Report

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

Project coverage is 90.81%. Comparing base (7ace826) to head (dd61f78).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1549 +/- ## ======================================= Coverage 90.81% 90.81% ======================================= Files 346 346 Lines 11063 11063 Branches 1815 1815 ======================================= Hits 10047 10047 Misses 669 669 Partials 347 347 ```

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