1115 Updating CEP18 Tutorials

[ACStone-MTS]

What does this PR fix/introduce?

This PR completely revamps the CEP18 tutorials.

Links have been fixed, code snippets updated and flow corrected so that it makes sense. I will open an additional ticket to update the Testing Framework tutorials, as the previous content was no longer accurate in any way.

Additional context

Update CEP18 Tutorials #1115

Checklist

(Delete any that aren't relevant)

• [x] Docs are successfully building - yarn install && yarn run build.
• [x] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• [x] All external links have been verified with yarn run check:externals.
• [x] My changes follow the Casper docs style guidelines.
• [x] All technical procedures have been tested (if you want help with this, mention it in Reviewers).

Reviewers

Writing and Flow: @ipopescu / Tech: @deuszex

[ACStone-MTS]

@ipopescu I'm planning to open another ticket to do the Testing Framework section. The recent updates made the existing tutorial completely incorrect, unfortunately.