Add MaterialX for MkDocs and update themes

[Andre601]

What kind of change does this PR introduce?

• Add a project
• Update a project
• Remove a project
• Add or update a category
• Change configuration
• Documentation
• Other, please describe:

Description:
This acts as a PR to update the Catalog, starting with adding the MaterialX for MkDocs project to the list.

Additionally are there apparently some issues with some projects (see recent workflow runs).

I'll leave this as a draft until the issues are resolved, allowing a clean workflow run.

Also, I would like @jaywhj input on this, as MaterialX for MkDocs is their project, so I don't want to publish any wrong info.

Checklist:

• I have read the CONTRIBUTING guidelines.
• I have not modified the README.md file. Projects are only supposed to be added or updated within the projects.yaml file since the README.md file is automatically generated.

[jaywhj]

@Andre601

Your PR update is correct. The issue with this workflow was caused by an incorrect configuration of another plugin, zettelkasten.

Below is the correct example you can update with:

- mkdocs_theme: zettelkasten-solarized-light
+ mkdocs_theme: zettelkasten

After making this change, the bot checks will pass.

Alternatively, you can first remove the configuration related to the zettelkasten plugin, this was an oversight from before; the zettelkasten author can add it back when needed.