Prepare "Developer Hub" and "Community" for Docusaurus v3 upgrade

[SebastianBezold]

We are planning to upgrade Docusaurus from v2.x to v3.x. This major version jump includes breaking changes.
There is a good guidance for preparing the website for this upgrade in the Docusaurus docs.

"Developer Hub" and "Community" do contain very basic content and should be prepared for the upgrade already

[SebastianBezold]

What I understood from the Docusaurus documentation:

• New version is parsing everything as MDX by default
• There is an option to opt for CommonMark for .md files. (format: md as frontmatter)
• In upcomming releases, the frontmatter might not be necessary, since docusaurus is planning to automatically use the right parser

With that info, I think we can get rid of most of the special character issues that are raised by the mdx-checker tool. The tool will still report it, since it is also interpreting everything as MDX, but the content will actually compile and also not produce runtime errors

[SebastianBezold]

Can't find the time right now and upgrade progress also seems to be on hold.