Docs and cleanup

[AnesBenmerzoug]

Description

This PR closes address parts of #462 and #489.

Changes

• Replace neoteroi plugin cards with mkdocs-material cards in the documentation home page.
• Make cards grow and change color just like in the transferlab website.
• Change title of first-steps.md from Getting Started to First Steps to match the filename.
• Add contributing guide to the documentation.
• Add a shorter version of the Applications of data valuation in machine learning blog post to the documentation with the help of Claude.
• Move Examples to their own section to make them more visible.
• Expand the table of content to make all sections visible.
• Set default documentation theme based on the system's preference.
• Installs memcached extra in all tox test environments (some notebooks were failing because of this).
• Remove outdated deprecation warnings.

Checklist

• [ ] ~Wrote Unit tests (if necessary)~
• [X] Updated Documentation (if necessary)
• [x] Updated Changelog
• [ ] ~If notebooks were added/changed, added boilerplate cells are tagged with "tags": ["hide"] or "tags": ["hide-input"]~

[AnesBenmerzoug]

I took care of the simple ones only in order to be able to finish this PR for the next release.

[AnesBenmerzoug]

I merged the two sections into one and then decided to remove more deprecated code and updated the tests accordingly.