Dark mode compatible logo

[lukruh]

Description

This is the PR to update the logo in the readme to be compatible with GH dark theme similar to the PR in the tool. The light and dark versions of the logo with transparent background are added to the docs folder, but I don't think the docs need to be changed in this Repo, do they?

Closes ESMValTool: 1406

Link to documentation:

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[codecov[bot]]

Codecov Report

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

Project coverage is 94.77%. Comparing base (ffaeff1) to head (2353613). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2532 +/- ## ======================================= Coverage 94.77% 94.77% ======================================= Files 249 249 Lines 14094 14094 ======================================= Hits 13358 13358 Misses 736 736 ```

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