Add extensions landing page

[katauber]

Add a landing page for extensions with minimal tutorials for each extension. Will resolve #2

[fsteeg]

Thanks, looks good already. Did some text tweaks, switched to using HTML image tags (and tweaked how the images are displayed), and added a Metafacture header image. Have a look at https://github.com/metafacture/metafacture.github.io/pull/3/commits/1825a5d32df0092a11c6d5157cd3abd22d3d8061 to see if these look good to you. Other than that, I noticed:

• The first image has an error in the editor
• First and second image have different coloring for strings
• With a collapsed side bar on the left side we'd focus more on the Fix/Flux file

[katauber]

Thank you for your feedback. I fixed the images of the fix and flux files. I am not really happy anymore having extensions/index.html in gitignore: gitignore has no effect because the file is added to the repository by GitHub Actions. Any ideas how to handle this?