refactor: introduce a shared folder for partials and images from both JS and Python courses

[honzajavorek]

Previously

• Python course had an images folder with images
• New JS course (unlisted) had an images folder as well, but it was just a symlink to the Python one
• Each of the courses had it's own _exercises.mdx

After this change

• There will be three top level folders: scraping_basics with shared stuff, scraping_basics_javascript2 for the JS course, scraping_basics_python for the Python course
• The scraping_basics contains shared images and partials (currently just _exercises.mdx, but there is a potential to share more in the future if we want)
• The JS and Python courses link to the shared folder for images and partials
• Only one shared _exercises.mdx, no duplication
• No symlinks or other hacks

Note

This change doesn't touch the original scraping_basics_javascript course at all.

[apify-service-account]

Preview for this PR was built for commit 8f903c5 and is ready at https://pr-1889.preview.docs.apify.com!