Update SpeedrunEthereum challenges in developers landing

[Pabl0cks]

We've just made a slight update to the SpeedrunEthereum Challenge 0 content and then realized that all the challenges showcased on the developers landing page would be token-related afterward. We believe this is a good time to update the other two showcased challenges, along with the content update from Challenge 0.

Description

• Updated the content from Challenge 0 on the /developers page and also in /developers/tutorials (externalTutorials).
• Replaced the Staking App and Create a Token challenges with DEX and Stablecoins.
• Updated DEX challenge name in /developers/tutorials (externalTutorials).
• Updated the challenge card badges to display challenge difficulty instead of the challenge number (showing challenges 0, 4, and 6 could look a bit odd).

Related Issue

Fixes #16426

Screenshot

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[wackerow]

Looks great, thanks @Pabl0cks!

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[wackerow]

@all-contributors please add @Pabl0cks for maintenance

[allcontributors]

@wackerow

I've put up a pull request to add @Pabl0cks! 🎉