Added xHydro logos

[lou-a]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Added a new logo for the xHydro repo and documentation.
• Added some CSS modifications so that the logos render well in both light and dark themes (courtesy of @SarahG-579462)

Does this PR introduce a breaking change?

No.

Other information:

This logo was designed by Élyse Fournier (HQ) and Louise Arnal (Ouranos), with inputs from the xHydro team.