Update MDX usage

[AdvancedCodingMonkey]

We use mdx 1.6, the current version is 3. The migration seems to not be straightforward however, which is why we kept it at version 1.6 so far. Usage of mdx seems to have changed.

One can also implement linting for mdx files, which I could not get to work so far. Here is how far I got:

1. Install eslint-plugin-mdx
2. Adding the mdx config and file extension in eslint.config.mjs:

import * as mdx from 'eslint-plugin-mdx'
...
importPlugin.flatConfigs.warnings,
importPlugin.flatConfigs.typescript,
jsxA11Y.flatConfigs.recommended,
mdx.flat,
...
parserOptions: {
    ecmaFeatures: {
      jsx: true,
    },

    project: ['./tsconfig.eslint.json'],
    warnOnUnsupportedTypeScriptVersion: true,
    extraFileExtensions: ['.md', '.mdx'],
  },

2. Adding file extensions to tsconfig.eslint.json:

"include": ["**/*.ts", "**/*.js", "**/*.tsx", "**/*.jsx", "**/*.mjs", "**/*.md", "**/*.mdx"]

=> eslint runs put results in parsing errors for .md and .mdx files.

Additionally, all our "mdx" files actually have a .md file extension, which we should probably change.

[AdvancedCodingMonkey]

It seems one also needs to import and add the mdx parser

import * as mdx from eslint-mdx

languageOptions:
   parser: mdx.parser

but this leads to more error mesages

[AdvancedCodingMonkey]

I did get mdx to work, just the linting remains.